Restore recently-pushed-branches-enhancements feature #6295

Description

• Following Drop recently-pushed-branches-enhancements feature #3753

We tried making that feature "look good" via CSS but we were limiting ourselves too much.

A better solution would be:

1. Load fragment on pages that don't have it, like before
2. Hide box via CSS (not display: none but more like .visuallyhidden, or else the next step won't work)
3. Detect it via selector-observer
4. Add a custom button somewhere in the header (e.g. a green PLUS next to the PRs tab)
5. When clicked, show the whole element unchanged

This means:

• We have total control over the display of the button
• We never have to alter the style of the actual branch list

Screenshot

No response

Example URLs

https://github.com/refined-github/refined-github (hide native box)
https://github.com/refined-github/refined-github/issues (load box and hide it)