Reserve system

I spent much too long removing reserved users from solr and then reverting those changes.

In any case, users who are in reserve are not sent saved search emails. I also added a check of in_reserve to User.can_receive_myjobs_email to hopefully prevent additional emails.

I may have missed a few bits and am open to suggestions.

All tests pass.
Passing in a user isn't really necessary as one is retrieved or created in save() already.