Introduce shared template for search inputs (#25338)

- Set
[type=search](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/search)
- Disable spellcheck
- Set maxLength 255 that I found in `templates/repo/issue/search.tmpl`
- Remove unnecessary `max-width`, it does nothing

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Giteabot <teabot@gitea.io>

templates/admin/base/search.tmpl

@@ -15,9 +15,9 @@
 		</div>
 	</div>
 </div>
-<form class="ui form ignore-dirty"  style="max-width: 90%;">
+<form class="ui form ignore-dirty">
 	<div class="ui fluid action input">
-		<input name="q" value="{{.Keyword}}" placeholder="{{.locale.Tr "explore.search"}}..." autofocus>
+		{{template "shared/searchinput" dict "locale" .locale "Value" .Keyword "AutoFocus" true}}
 		<button class="ui primary button">{{.locale.Tr "explore.search"}}</button>
 	</div>
 </form>