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/repo/issue/milestones.tmpl

@@ -28,7 +28,7 @@
 			<form class="list-header-search ui form ignore-dirty">
 				<div class="ui small search fluid action input">
 					<input type="hidden" name="state" value="{{$.State}}">
-					<input name="q" value="{{.Keyword}}" placeholder="{{.locale.Tr "explore.search"}}...">
+					{{template "shared/searchinput" dict "locale" .locale "Value" .Keyword}}
 					<button class="ui small icon button" type="submit" aria-label="{{.locale.Tr "explore.search"}}">
 						{{svg "octicon-search"}}
 					</button>