Issue 265 blue login problem

This fixes #265.  @devgeeks, @merickson, we probably should give priority to releasing this as soon as possible - maybe through a Play store beta release, if we can get that working.

It turns out that [enterprise (blue)](https://spideroak.com/business/) service use a completely different username - the one base32-encoded in the content URLs - than the one that is used for login.  Our provisions to refuse the server's acceptance of login usernames that differed from the account username by alphabetic case was not allowing that.  I've narrowed the provision to only disallow variation by alphabetic case, and blue logins are now working.