Rework header bar on issue, pull requests and milestone (#24420)

- Make search bar dynamic full width via flexbox
- Make all buttons `small` so font size is the same for all elements in
the header
- Remove primary color from search field, add SVG icon like on Code tab
- Fix button vertical padding being enlarged by SVG icons

[View diff without
whitespace](https://github.com/go-gitea/gitea/pull/24420/files?diff=unified&w=1)

<img width="1226" alt="Screenshot 2023-04-29 at 11 58 53"
src="https://user-images.githubusercontent.com/115237/235296851-74848267-664f-4c1f-b94c-a1b94196ff75.png">
<img width="1219" alt="Screenshot 2023-04-29 at 11 59 39"
src="https://user-images.githubusercontent.com/115237/235296852-bcfde5ed-8658-43c2-b7e5-3ad84611e76f.png">

Mobile:
<img width="437" alt="Screenshot 2023-04-29 at 11 59 52"
src="https://user-images.githubusercontent.com/115237/235296860-99263373-7b27-4540-868c-a93e70f281ca.png">
<img width="433" alt="Screenshot 2023-04-29 at 12 00 00"
src="https://user-images.githubusercontent.com/115237/235296862-6cf64317-a864-405a-a00f-b5ab620349f5.png">

templates/repo/issue/search.tmpl

@@ -1,5 +1,5 @@
-<form class="ui form ignore-dirty">
-	<div class="ui search fluid action input">
+<form class="list-header-search ui form ignore-dirty">
+	<div class="ui small search fluid action input">
 		<input type="hidden" name="type" value="{{$.ViewType}}">
 		<input type="hidden" name="state" value="{{$.State}}">
 		<input type="hidden" name="labels" value="{{.SelectLabels}}">
@@ -8,6 +8,8 @@
 		<input type="hidden" name="assignee" value="{{$.AssigneeID}}">
 		<input type="hidden" name="poster" value="{{$.PosterID}}">
 		<input name="q" value="{{.Keyword}}" placeholder="{{.locale.Tr "explore.search"}}...">
-		<button class="ui primary button" type="submit">{{.locale.Tr "explore.search"}}</button>
+		<button class="ui small icon button" type="submit" aria-label="{{.locale.Tr "explore.search"}}">
+			{{svg "octicon-search"}}
+		</button>
 	</div>
 </form>