DEV: Use Ember router to determine currentURL

This means that our DiscourseURL logic will work consistently in tests, where `window.location` doesn't get updated.

To make it work properly, our `replaceState` implementation needed to be updated so that it writes the new URL to Ember's router, rather than bypassing the router and going straight to the `location` API.

A couple of tests needed updating following this fix:
- the composer-test was asserting that the new reply should be missing from the DOM... when really it **should** be in the DOM, and this fix to the test environment makes it so
- the topic-test was making a fake topic fixture based on the data from a topic with a different id. This was causing the topic route to get confused, and 'fix' the currentURL. This commit updates it to use a fixture with consistent data.

This commit also removes the feature detection of `window.history`. It's feature-detected within `discourse-location`. Plus, we don't support any browsers without it.

app/assets/javascripts/discourse/app/lib/url.js

@@ -178,22 +178,15 @@ const DiscourseURL = EmberObject.extend({
     });
   },
 
-  // Browser aware replaceState. Will only be invoked if the browser supports it.
   replaceState(path) {
-    if (
-      window.history &&
-      window.history.pushState &&
-      window.history.replaceState &&
-      window.location.pathname !== path
-    ) {
+    if (this.router.currentURL !== path) {
       // Always use replaceState in the next runloop to prevent weird routes changing
       // while URLs are loading. For example, while a topic loads it sets `currentPost`
       // which triggers a replaceState even though the topic hasn't fully loaded yet!
       next(() => {
-        const location = this.get("router.location");
-        if (location && location.replaceURL) {
-          location.replaceURL(path);
-        }
+        // Using the private `_routerMicrolib` is not ideal, but Ember doesn't provide
+        // any other way for us to do `history.replaceState` without a full transition
+        this.router._routerMicrolib.replaceURL(path);
       });
     }
   },
@@ -249,7 +242,8 @@ const DiscourseURL = EmberObject.extend({
       return this.replaceState(path);
     }
 
-    const oldPath = `${window.location.pathname}${window.location.search}`;
+    const oldPath = this.router.currentURL;
+
     path = path.replace(/(https?\:)?\/\/[^\/]+/, "");
 
     // Rewrite /my/* urls

app/assets/javascripts/discourse/tests/acceptance/composer-test.js

@@ -202,7 +202,7 @@ acceptance("Composer", function (needs) {
     await click("#reply-control button.create");
     assert.strictEqual(
       queryAll(".cooked:last p").text(),
-      "If you use gettext format you could leverage Launchpad 13 translations and the community behind it."
+      "this is the content of my reply"
     );
   });
 

app/assets/javascripts/discourse/tests/acceptance/topic-test.js

@@ -586,19 +586,14 @@ acceptance("Navigating between topics", function (needs) {
     const firstPost = topicResponse.post_stream.posts[0];
     firstPost.cooked += `\n<a class='same-topic-slugless' href='/t/280'>Link 1</a>`;
     firstPost.cooked += `\n<a class='same-topic-slugless-post' href='/t/280/3'>Link 2</a>`;
-    firstPost.cooked += `\n<a class='diff-topic-slugless' href='/t/281'>Link 3</a>`;
-    firstPost.cooked += `\n<a class='diff-topic-slugless-post' href='/t/281/3'>Link 3</a>`;
+    firstPost.cooked += `\n<a class='diff-topic-slugless' href='/t/28830'>Link 3</a>`;
+    firstPost.cooked += `\n<a class='diff-topic-slugless-post' href='/t/28830/1'>Link 4</a>`;
     firstPost.cooked += `\n<a class='by-post-id' href='/p/${firstPost.id}'>Link to Post</a>`;
 
     server.get("/t/280.json", () => helper.response(topicResponse));
     server.get("/t/280/:post_number.json", () =>
       helper.response(topicResponse)
     );
-
-    server.get("/t/281.json", () => helper.response(topicResponse));
-    server.get("/t/281/:post_number.json", () =>
-      helper.response(topicResponse)
-    );
   });
 
   test("clicking slug-less URLs within the same topic", async function (assert) {
@@ -613,10 +608,11 @@ acceptance("Navigating between topics", function (needs) {
   test("clicking slug-less URLs to a different topic", async function (assert) {
     await visit("/t/-/280");
     await click("a.diff-topic-slugless");
-    assert.ok(currentURL().includes("/281"));
+    assert.ok(currentURL().includes("/28830"));
 
+    await visit("/t/-/280");
     await click("a.diff-topic-slugless-post");
-    assert.ok(currentURL().includes("/281"));
+    assert.ok(currentURL().includes("/28830"));
   });
 
   test("clicking post URLs", async function (assert) {

app/assets/javascripts/discourse/tests/unit/lib/url-test.js

@@ -77,6 +77,11 @@ module("Unit | Utility | url", function () {
     logIn();
     const user = User.current();
 
+    sinon.stub(DiscourseURL, "router").get(() => {
+      return {
+        currentURL: "/forum",
+      };
+    });
     sinon.stub(DiscourseURL, "handleURL");
     DiscourseURL.routeTo("/my/messages");
     assert.ok(