Homogenize sort and filter dropdown toggles

## What does this MR do?

Homogenizes the style of *filter* and *sort* dropdown toggles (aka buttons) related to:
- *issues* (index, new & edit views)
- *issue boards* (index);
- *merge requests* (index, new, edit);
- *projects* (index and "explore");
- *projects admin* (index);
- *groups* (show and "explore");
- *groups admin* (index);
- *users admin* (index);
- *todos* (index);
- *branches* (index);
- *commits* (index);
- *cycle analytics*;
- *network*;
- *forks*;
- *tags* (index);
- *global search*

Some other dropdowns are incidently concerned:
- project admin (show);
- assigne/due date/milestone/label dropdowns on the issue and board sidebars;
- stage dropdown on the build sidebar;
- merge request and comparison dropdowns for choosing forks and branches;
- ref switcher (e.g. in the blob (show) view or in the graphs view);

Dropdown toggles concerned by #24150 but not covered in this MR:
- Dropdowns for selection MR versions to be compared (in the "Changes" tab of an MR);
- Dropdowns that bring up a list of actions:
  - "Settings" dropdowns (such as the one found on the upper-right corner of all project pages);
  - The download/action/notifications buttons on the main project page;
  - Dropdowns located inside of list items (on list of pipelines, users and groups, for instance);
  - The "Options" button on the commit (show) view;
  - The "+" button on the tree explorer (for creating files, branches, tags, ...)

### TODO
- [ ] Update ui.html if needed

## Are there points in the code the reviewer needs to double check?

Am I changing any other dropdown toggle than listed above?

## Why was this MR needed?

For #24150

## Screenshots (if relevant)

### Issue Index

**Before:**
![issue-index-before](/uploads/1aa358b0b9e79fd3a7467b57cd2b03ec/issue-index-before.png)

**After:**
![issue-index-after](/uploads/7a4159e8927d456ed3f4390072174e49/issue-index-after.png)

### Issue New

**Before:**
![issue-new-before](/uploads/fcfb9c39d2ff8f00827f52d60db5c114/issue-new-before.png)

**After:**
![issue-new-after](/uploads/a0b151d257d8f3e8f98a2873427362ee/issue-new-after.png)

### Merge Request Branch Selector

**Before:**
![merge-request-new-before](/uploads/8dd7b6e8ea38af7aced743386bfb83ca/merge-request-new-before.png)

**After:**
![merge-request-new-after](/uploads/38ad8f5a315344b3bab7e936a84acf7e/merge-request-new-after.png)

### Global Search
![global-search-after](/uploads/f2912b7d91fd286028cb9481d2d53a82/global-search-after.png)

### Cycle Analytics
![cycle-analytics-after](/uploads/248d5ca360bb510aa0f0bfa8a1a00aae/cycle-analytics-after.png)


## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~API support added~~
- Tests
  - ~~Added for this feature/bug~~
  - [ ] All builds are passing
- ~~Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)~~
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

#24150

See merge request !7583