Issue 3034: Invitations link for non-existing user causes error 500

Resolves issue: http://code.google.com/p/otwarchive/issues/detail?id=3034

http://www.archiveofourown.org/users/SOMEUSERNAME/invitations

Changed invitations controller to check to make sure that the SOMEUSERNAME is a real user. If not, it redirects to the log in page (if the current_user is not logged in). 

If the current_user is logged in, controller already redirects the current_user to their user dashboard (no rails error was showing).