shell: Disables realms-op-auth button

when joining a domain, the realms-op-auth select picker is disabled

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Displays waiting text and fixes spacing

Displays text to inform the user that the transaction may take a while
and also fixes bug which made the cancel button go above the join button

Fixes #918
Closes #938
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Hides throbber and text after domain join cancelled

Fixes bug that shows throbber and join message when canceling during
joining a domain. Also exits realm join window when the domain is joined
to get rid of the delay which allowed users to cancel after the domain
was joined.

Reviewed-by: Stef Walter <stefw@redhat.com>