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 (moved) 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 (moved)

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?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#24150 (moved)