Use different user.construct_public_followings_sql query depending on database backend.
This is done for performance reasons as a temporary fix for problems with 0.4.0.0 on MySQL. This madness is explained in issue https://github.com/diaspora/diaspora/issues/5014.
Merge pull request #5026 from jaywink/hotfix-mysql-madness-develop

Use different user.construct_public_followings_sql query depending on database backend