[#1178] Remove backward migrations, as we don't support it
[#1178] Fix repeated test name
[#1178] Use the data returned by validate, instead of the one passed as param
[#1178] Move user_invite_schema to ckan.logic.schema
[#1178] Refactor user name creation

I've isolated the loop where it needs to be. This makes the code clearer, but
makes it susceptible to errors. For example, after the random name was
generated but before the user is created, another user with the same name might
be created, breaking our code.

This should be rare enough to bother us.