Backport #30238 by @silverwind
Create a new `issue-navbar` class specifically for this bar, previous
class used in many places and I thought I had them all removed, but not
this one.
Fixes: https://github.com/go-gitea/gitea/issues/30226
Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit 39322d9fc4644d2ede3efe3a9c4efee2e493d4cd)