Repo branch page: label size, PR ref, new PR button alignment

[CL-Jeremy]

1. The orange label (for "already included in default branch") was left out in Branch page and misc css improvements #15208
2. The "New Pull Request" button has a right margin that doesn't align with the above labels
3. Reference links to pull requests should trigger preview pop-ups

This PR fixes the above tiny bits.

I'll try my best to backport this and #15276 now.

[silverwind]

Reference links to pull requests should trigger preview pop-ups

I think the approach of having to add classes to every link is not ideal. A purely JS-based link detection would surely work better without burdening us to add those classes everywhere. Thought, adding it is fine for now.

[lunny]

Could you post a screenshot comparison?

[codecov-commenter]

Codecov Report

Merging #15363 (0937248) into main (143071e) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #15363      +/-   ##
==========================================
+ Coverage   43.98%   44.02%   +0.04%     
==========================================
  Files         681      681              
  Lines       82230    82230              
==========================================
+ Hits        36168    36202      +34     
+ Misses      40143    40123      -20     
+ Partials     5919     5905      -14     

Impacted Files	Coverage Δ
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
models/repo_list.go	77.04% <0.00%> (-0.78%)	⬇️
routers/repo/view.go	40.66% <0.00%> (-0.59%)	⬇️
modules/notification/mail/mail.go	38.77% <0.00%> (ø)
models/error.go	39.84% <0.00%> (+0.78%)	⬆️
models/notification.go	66.07% <0.00%> (+0.88%)	⬆️
modules/log/event.go	59.90% <0.00%> (+0.94%)	⬆️
services/gitdiff/gitdiff.go	73.76% <0.00%> (+1.41%)	⬆️
services/pull/patch.go	55.93% <0.00%> (+1.69%)	⬆️
modules/notification/ui/ui.go	65.81% <0.00%> (+1.70%)	⬆️
... and 4 more

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