Improve notification icon and navbar (#25111)
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="https://github.com/go-gitea/gitea/assets/115237/23cdc3d6-7f63-49df-bec3-f2e75e32a304">
<img width="63" alt="Screenshot 2023-06-07 at 00 07 14"
src="https://github.com/go-gitea/gitea/assets/115237/fae602c2-4467-4d50-b1ec-56317843f9a2">
<img width="84" alt="Screenshot 2023-06-07 at 00 07 36"
src="https://github.com/go-gitea/gitea/assets/115237/c48141b8-0b3c-48cc-846a-3a272524dbdb">
<img width="329" alt="Screenshot 2023-06-07 at 00 25 10"
src="https://github.com/go-gitea/gitea/assets/115237/cda612f1-426e-466b-a351-fc992bfd18fd">
<img width="186" alt="Screenshot 2023-06-07 at 00 35 45"
src="https://github.com/go-gitea/gitea/assets/115237/04484a2e-9bbf-493c-aa26-8e936da008fa">
<img width="797" alt="Screenshot 2023-06-07 at 16 57 40"
src="https://github.com/go-gitea/gitea/assets/115237/e7ccb672-5807-4cb6-b306-b18ae0c7e321">
silverwind