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:**
**After:**
### Issue New
**Before:**
**After:**
### Merge Request Branch Selector
**Before:**
**After:**
### Global Search
### Cycle Analytics
## 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
63f5c4ea54