Factor out an isIdAvailable() function from EditForm.clean_id() in
soc/views/site/user/profile.py and add it to soc/logic/site/id_user.py.
Addresses some comments on r614.
Patch by: Todd Larsen
Review by: Pawel Solyga
Review URL: http://codereviews.googleopensourceprograms.com/1202