In itself, this change will not cause tests to run in parallel. It just unlocks the ability to use tools like `ember exam` to run tests in parallel. For example:
```
yarn ember exam --load-balance --parallel=3 --random
```
We previously had a system which would generate a 10x10px preview of images and add their URLs in a data-small-upload attribute. The client would then use that as the background-image of the `<img>` element. This works reasonably well on fast connections, but on slower connections it can take a few seconds for the placeholders to appear. The act of loading the placeholders can also break or delay the loading of the 'real' images.
This commit replaces the placeholder logic with a new approach. Instead of a 10x10px preview, we use imagemagick to calculate the average color of an image and store it in the database. The hex color value then added as a `data-dominant-color` attribute on the `<img>` element, and the client can use this as a `background-color` on the element while the real image is loading. That means no extra HTTP request is required, and so the placeholder color can appear instantly.
Dominant color will be calculated:
1. When a new upload is created
2. During a post rebake, if the dominant color is missing from an upload, it will be calculated and stored
3. Every 15 minutes, 25 old upload records are fetched and their dominant color calculated and stored. (part of the existing PeriodicalUpdates job)
Existing posts will continue to use the old 10x10px placeholder system until they are next rebaked
Upgrading to Markdown.it v13 broke empty inline BBCodes. This works around the problem by adding an empty token before a closing token if the previous token was a BBCode token.
It also removes the unused `jump` attribute which was removed in Markdown.it v12.3
The experimental user menu has a tab that displays recent reviewables and at the moment when a new signs up for the site and they need to be approved, admins see a very scary "suspicious user" copy in the reviewables tab in the user menu. We don't need the copy to be very scary because when a user needs to be approved, it's because the site operator has configured the site to force all new users to go through the review queue and it's not some kind of spam detector flagging the user.
When tagging is enabled, we were correctly serializing tags by their name. However, when tagging was disabled we were attempting to serialize an entire Tag object which raises an error since ee07f6da7d.
https://meta.discourse.org/t/232885
The key fix in this commit is that it removes `this.replaceState(path)` for anchor-only URLs. We still intercept those routing changes to properly calculate the scroll position of the anchor via `jumpToElement`, but we no longer use the Ember router to override the browser's history. This fixes the subfolder issue and also lets the browser maintain its history correctly.
The commit also includes a small refactor to the `jumpToElement` helper to facilitate stubbing in tests.
Secure media requests go through the app. In topics with many images, this makes it very easy to hit rate limiters. Skipping the low-res placeholders reduces the chance of this problem occuring.
Reverts #18241 and fixes issues with the original PR:
1. Remove an extraneous `margin-left: auto` from a grid cell (this was causing the buggy behavior in webkit)
2. Add `grid-area` name to `.extra-info-wrapper`
3. Account for `.wrap` padding
4. Remove unused css (`.header-row` and inner styles)
