Fix missing affixed sidebar offset

[YUCLing]

Fixes #4122

Changes proposed in this pull request:
Switched to sticky positioning and fixed the missing offset by backporting #4111. Bootstrap's affix plugin is kept for compatibility.

Reviewers should focus on:

Screenshot

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)

[SychO9]

Hi @YUCLing thanks for the PR!

Looking at the issue, it seems easier to just add the needed margin when the nav is fixed in place: #4147

[YUCLing]

Looking at the issue, it seems easier to just add the needed margin when the nav is fixed in place: #4147

Well, this also switches to a more modern way to make the nav fixed. Still, it's up to you to decide which way to take.

[SychO9]

I'd definitely like to see us replacing jquery/bootstrap, but since that won't happen in the 2.x line anyway, my preference is simpler changes especially as we're looking to stabilize 2.0 right now.

Still, this will be a great reference for v3 when we get to dropping jquery and bootstrap!

Closing for now. Thanks again!