Make daily emails respect unsubscribe status [MJ-473]

- Re-add opt_in_myjobs to account form
- Add new options to daily emails
- Create a new view decorator that prevents some unauthorized access
- Make saved search digests update the last_sent member of a user's saved searches
- Improve sending of daily emails - respect send_if_none and opt_in_myjobs, among other fixes

@michaelgzoller is back from being sick. This pull request is my gift to you.

All tests pass.