By using git's ability, end users (especially small instance users) do
not need to enable the indexer, they could also benefit from the code
searching feature.
Fix#29996


---------
Co-authored-by: silverwind <me@silverwind.io>
Likely still a few useless classes left, but I think I at least don't
have missed any.
---------
Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Giteabot <teabot@gitea.io>
Unify all but a few search boxes to use uniform style, uniform
translations and shared templates where possible.
Remove a few duplicated search templates, e. g. code search.
<details><summary>Example after screenshots:</summary>




</details>
Also includes #29700
Co-authored-by: 6543 <6543@obermui.de>
---------
Co-authored-by: 6543 <m.huber@kithara.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>