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">

web_src/css/repository.css

@@ -161,7 +161,7 @@
 
 .repository .ui.tabs.divider {
   margin-top: -1px;
-  margin-bottom: 20px;
+  margin-bottom: 12px;
 }
 
 .repository #clone-panel #repo-clone-url {
@@ -3289,18 +3289,6 @@ td.blob-excerpt {
   .repository.file.list #repo-files-table .commit-list span.commit-summary {
     display: none !important;
   }
-  .issue-list-headers.ui[class].grid > div:nth-child(1) {
-    order: 1;
-    width: 50%;
-  }
-  .issue-list-headers.ui[class].grid > div:nth-child(2) {
-    order: 3;
-    width: 100%;
-  }
-  .issue-list-headers.ui[class].grid > div.column:not(.row):nth-child(3) {
-    order: 2;
-    width: 50%;
-  }
   .repository.view.issue .comment-list .timeline,
   .repository.view.issue .comment-list .timeline-item {
     margin-left: 0;