Improve AutofollowUsers#auto_followers performance
Improve AutofollowUsers#auto_followers performance

Replace a loop over all users with the equivalent SQL query.

One disadvantage of this is that the default value must now be a
constant (the default value for `user_preferences`.`auto_follow_topics`)

Refs #807

thredded/thredded,thredded/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded