Introducing: Developer navigation bar

In line with the code blog and other dev sites (Wiki, DevTalk)

Everything is pretty much copied from the one in blender.org (_navbar_developer.php),
with some additions to make it work in Gitea.

The styling additions are in the <style> block with the comment "/* Custom styling for Gitea */"

The link to "Projects" is projects.blender.org (since it makes sense here but
not yet in the Code blog).

On the Gitea side, two custom templates have changed:
* head_navbar.tpml was changed to add the mobile navbar expand toggle.
* body_inner_pre.tpml contains the actual navbar, copied from the blender.org repo
  with the following changes:
 * Added <style> block custom for Gitea.
 * Added custom notifications code from head_navbar.tpml
 * Added user buttons (notif, create, register, sign in, etc),
   See the area around "{{/* Here starts the custom styling for gitea. */}}"
 * In order to style the buttons as such, the class `navbardev-btn` was added
   to the Register, Sign In, Notifications, Create, and User dropdown buttons.

templates/base/head_navbar.tmpl

@@ -1,4 +1,10 @@
-<div class="ui container fluid py-3">
+<div class="ui container" id="navbar">
+  <div class="item brand mobile-only">
+    <div class="ui basic icon button mobile-only" id="navbar-expand-toggle">
+      <i class="sidebar icon"></i>
+    </div>
+  </div>
+
 	{{if and .IsSigned .MustChangePassword}}
 		{{/* No links */}}
 	{{else if .IsSigned}}