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">

tests/integration/pull_compare_test.go

@@ -19,7 +19,7 @@ func TestPullCompare(t *testing.T) {
 	req := NewRequest(t, "GET", "/user2/repo1/pulls")
 	resp := session.MakeRequest(t, req, http.StatusOK)
 	htmlDoc := NewHTMLParser(t, resp.Body)
-	link, exists := htmlDoc.doc.Find(".ui.three.column.grid").Find(".ui.green.button").Attr("href")
+	link, exists := htmlDoc.doc.Find(".new-pr-button").Attr("href")
 	assert.True(t, exists, "The template has changed")
 
 	req = NewRequest(t, "GET", link)