Markdown and Repo header tweaks (#13744)

* Markdown and Repo header tweaks

- Use CSS vars for all markdown colors
- Tweak repo header, removing double borders and adjust sizes
- Use menu instead of buttons for issue open/close switcher
- Add emoji inversion for select emoji glyphs in arc-green
- Use border over box-shadow for all buttons
- Add spacing element to login form without openid

* repo settings navbar fix

* use shared template in more places and adjust dashboard

* fix remaining open/close combos

templates/user/auth/signin_navbar.tmpl

@@ -18,4 +18,6 @@
 		{{end}}
 	</div>
 </div>
+{{else}}
+	<div class="mt-4"></div>
 {{end}}