Disable signup at the controller level.

This is a temporary measure during beta stage. During beta, users will only join via invitation.

Signup will be enabled again when beta stage ends.
Disable signup form at the view level. Also make it look faded.

This disables signup while the app is in the beta stage. Users will only be able to sign up through an invitation.
Removed unused CSS class "alert-block" from views.
Added warning message about the closed beta to the disabled signup form.
Commented out tests related to the currently disabled sign up feature.

They will be uncommented when the beta stage ends and signup is opened again for everyone.