feat: use position: sticky for discussion side nav

[davwheat]

Fixes #3541

Changes proposed in this pull request:

Swaps position: fixed for position: sticky for the Discussion sidebar. This helps to increase the amount of screen real estate for extension to add controls to the sidebar, when a large discussion hero is used.

Reviewers should focus on:

Removal of the overflow-x: hidden is needed for sticky to work correctly. This shouldn't pose any issues for us, but bad extensions could cause x overflow to occur now.

We could keep it, but set to visible when the selector is .App.App-discussion instead.

Screenshot

MIQPCApG.mp4

GxNZkrpE.mp4

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[SychO9]

🤩

[SychO9]

Looks good!