DEV: Use RouteTemplate for admin API keys template (#30835)

This is a small clean-up PR that does the following:

- Convert api-keys.hbs to a RouteTemplate backed api-keys.gjs.
- Move the sub-page templates (index, show, new) into /api-keys sub-directory.
- Removes some styles that aren't used after the admin UI conversion.

app/assets/javascripts/admin/addon/templates/api-keys.gjs

@@ -1,5 +1,11 @@
+import RouteTemplate from "ember-route-template";
+import DBreadcrumbsItem from "discourse/components/d-breadcrumbs-item";
+import DPageHeader from "discourse/components/d-page-header";
+import PluginOutlet from "discourse/components/plugin-outlet";
+import { i18n } from "discourse-i18n";
+
+export default RouteTemplate(<template>
   <PluginOutlet @name="admin-api-keys">
-  <div class="admin-api-keys admin-config-page">
     <DPageHeader
       @titleLabel={{i18n "admin.api_keys.title"}}
       @descriptionLabel={{i18n "admin.api_keys.description"}}
@@ -23,5 +29,5 @@
     <div class="admin-container admin-config-page__main-area">
       {{outlet}}
     </div>
-  </div>
   </PluginOutlet>
+</template>);

app/assets/stylesheets/common/admin/api.scss

@@ -25,55 +25,7 @@
 
 // Api keys
 
-.d-admin-table.api-keys {
-  margin-bottom: 0.25em;
-
-  tr.revoked {
-    color: var(--primary-high);
-  }
-
-  .d-admin-row__overview.key {
-    width: 20%;
-  }
-}
-
 .admin-api-keys {
-  h2 {
-    margin-bottom: 10px;
-  }
-  .api-key-new {
-    .form-element,
-    .form-element-desc {
-      &.input-area {
-        width: 100%;
-
-        .value-list,
-        .select-kit,
-        input[type="text"],
-        input[type="text"].filter-input {
-          width: 100%;
-          margin: 0;
-        }
-
-        @include breakpoint(mobile-extra-large, min-width) {
-          width: 75%;
-          .value-list,
-          .select-kit,
-          input[type="text"] {
-            width: 50%;
-          }
-        }
-      }
-      &.label-area {
-        label {
-          margin: 0.5em 1em 0 0;
-        }
-        @include breakpoint(mobile-extra-large, min-width) {
-          width: 25%;
-        }
-      }
-    }
-  }
   .api-key-show {
     .form-element,
     .form-element-desc {