Unify search boxes (#29530)

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>


![grafik](https://github.com/go-gitea/gitea/assets/47871822/e20e7d6b-c6be-4a47-b132-672766f41421)


![grafik](https://github.com/go-gitea/gitea/assets/47871822/d5b11b9c-c12f-4a29-8fb0-24e5aa511d18)


![grafik](https://github.com/go-gitea/gitea/assets/47871822/d86bb444-36c7-426d-9cf1-c634963dffb1)


![grafik](https://github.com/go-gitea/gitea/assets/47871822/a76c0319-0518-484a-a840-563d02b61198)

</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>

web_src/js/components/DashboardRepoList.vue

@@ -355,7 +355,7 @@ export default sfc; // activate the IDE's Vue plugin
         </a>
       </h4>
       <div class="ui attached segment repos-search">
-        <div class="ui fluid action left icon input" :class="{loading: isLoading}">
+        <div class="ui small fluid action left icon input" :class="{loading: isLoading}">
           <input type="search" spellcheck="false" maxlength="255" @input="changeReposFilter(reposFilter)" v-model="searchQuery" ref="search" @keydown="reposFilterKeyControl" :placeholder="textSearchRepos">
           <i class="icon"><svg-icon name="octicon-search" :size="16"/></i>
           <div class="ui dropdown icon button" :title="textFilter">