Improve notification icon and navbar (#25111) (#25124)

Backport #25111 by @silverwind

Improvements to the notification icon and `<nav>`:

- Add a opaque color for header hover and use it, allowing the border to
be the right color on hover (sadly, not otherwise possible with CSS, not
even `color-mix`).
- Increase font size by 1px
- Use flexbox for slightly better text centering
- Reduce padding of user and add repo button, add margin on right side
of user menu
- Remove the `following bar` wrapper on navbar

<img width="176" alt="Screenshot 2023-06-07 at 00 07 08"
src="23cdc3d6-7f63-49df-bec3-f2e75e32a304">
<img width="63" alt="Screenshot 2023-06-07 at 00 07 14"
src="fae602c2-4467-4d50-b1ec-56317843f9a2">
<img width="84" alt="Screenshot 2023-06-07 at 00 07 36"
src="c48141b8-0b3c-48cc-846a-3a272524dbdb">
<img width="329" alt="Screenshot 2023-06-07 at 00 25 10"
src="cda612f1-426e-466b-a351-fc992bfd18fd">
<img width="186" alt="Screenshot 2023-06-07 at 00 35 45"
src="04484a2e-9bbf-493c-aa26-8e936da008fa">
<img width="797" alt="Screenshot 2023-06-07 at 16 57 40"
src="e7ccb672-5807-4cb6-b306-b18ae0c7e321">

---------

Co-authored-by: silverwind <me@silverwind.io>

web_src/css/repo.css

@@ -132,7 +132,7 @@
 }
 
 .repository .header-wrapper {
-  background-color: var(--color-navbar);
+  background-color: var(--color-header-wrapper);
 }
 
 .repository .header-wrapper .ui.tabs.divider {