authPrompt: give message label an initial style

This commit consolidates the styles of the various
message types into one 'login-dialog-message' style
and then adds additional styles on top to cover the
differences.

This allows us to give the message label an initial
style so that is padded properly before any messages
are displayed.

https://bugzilla.gnome.org/show_bug.cgi?id=706670
theme: add bottom badding for login dialog hint

Since we're getting rid of the top padding, we need to add what we
take away to the bottom padding to keep the size the same.

https://bugzilla.gnome.org/show_bug.cgi?id=706670