Fix mobile navbar and misc cleanups (#25134) (#25169)

Backport #25134 by @silverwind

- Fix and improve mobile navbar layout
- Apply all cleanups suggested in
https://github.com/go-gitea/gitea/pull/25111
- Make media query breakpoints match Fomantic's exactly
- Clean up whitespace in class on navbar items

Mobile navbar before and after:
<img width="745" alt="Screenshot 2023-06-08 at 08 40 56"
src="https://github.com/go-gitea/gitea/assets/115237/ca84b239-b10f-41db-8c06-dcf2b6dd9d28">
<img width="739" alt="Screenshot 2023-06-08 at 08 41 23"
src="https://github.com/go-gitea/gitea/assets/115237/09133c54-eb7e-4110-858c-ead23c3b7521">

web_src/css/repo/linebutton.css

@@ -16,7 +16,7 @@
   position: absolute;
   font-family: var(--fonts-regular);
   left: 0;
-  transform: translateX(-70%);
+  transform: translateX(-50%);
   cursor: pointer;
 }
 

web_src/css/repo/release-tag.css

@@ -119,7 +119,7 @@
   }
 }
 
-@media (max-width: 767px) {
+@media (max-width: 767.98px) {
   .repository.new.release .field button {
     margin-bottom: 1em;
   }