Theming polish

* Fix selector for removing the top spacing added to the body by
  admin menu.
* Correctly detect if a logo is present to select the width of the
  main container.
* Give the left container a class, sidebar-first-container, instead
  of using its grid class as a selector.

Addresses #3551
Add "Clerk Login" link to user menu

Add login link (links to user/login) with label "Clerk Login" to
user menu.

Addresses #3551