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

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

templates/shared/searchinput.tmpl

@@ -0,0 +1 @@
+<input type="search" spellcheck="false" name="q" maxlength="255" placeholder="{{.locale.Tr "explore.search"}}…"{{if .Value}} value="{{.Value}}"{{end}}{{if .AutoFocus}} autofocus{{end}}{{if .Disabled}} disabled{{end}}>