Make isIdDeveloper() also able to check an is_developer Boolean property in
User entities.
Review URL: http://codereviews.googleopensourceprograms.com/601
Replace checkLinkNameForId() with isLinkNameAvailableForId(), and implement
that new function in terms of only the existing doesLinkNameExist() and
doesLinkNameBelongToId().
Patch by: Todd Larsen
Review by: Pawel Solyga
Review via: discussion of patches on melange-soc-dev mailing list