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/list-header.css

@@ -0,0 +1,36 @@
+.list-header {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: .5rem;
+}
+
+.list-header-sort {
+  display: flex;
+  align-items: center;
+  justify-content: flex-end;
+  padding-left: 1rem;
+  padding-right: 1rem;
+}
+
+.list-header-search {
+  display: flex;
+  flex: 1;
+  align-items: center;
+  flex-wrap: wrap;
+  justify-content: center;
+  min-width: 200px; /* to enable flexbox wrapping on mobile */
+}
+
+.list-header-search .input {
+  flex: 1;
+}
+
+.small-pill-buttons {
+  min-height: 35.4px !important; /* match .small.button in height */
+}
+
+.small-pill-buttons .item {
+  padding-top: 6px !important;
+  padding-bottom: 6px !important;
+}