Fix mobile navbar and misc cleanups (#25134)

- 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">

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

templates/user/dashboard/navbar.tmpl

@@ -77,7 +77,7 @@
 		{{end}}
 
 	{{if .ContextUser.IsOrganization}}
-		<div class="right stackable menu">
+		<div class="right menu">
 			<a class="{{if .PageIsNews}}active {{end}}item gt-ml-auto" href="{{.ContextUser.DashboardLink}}{{if .Team}}/{{PathEscape .Team.Name}}{{end}}">
 				{{svg "octicon-rss"}}&nbsp;{{.locale.Tr "activities"}}
 			</a>