fix: unread count in post stream not visible

[SychO9]

Fixes #3642

Changes proposed in this pull request:
Removes the overflow hidden CSS rule. Applies a 0 opacity when the unread is 0.
Visually the unread label never seems to overlap with the text above it, so it seems fine.

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)
• Related core extension PRs: (Remove if irrelevant)

[askvortsov1]

Whoah, simple fix for a very old bug!

Maybe we should use css visibility for semantic simplicity? Or conditionally add display: none?

[SychO9]

unfortunately, visibility and display just don't work out well, maybe because of how jQuery does the animation.