Send weekly warning dmails to approvers in danger of losing their
approver permissions. Don't send warnings if we're more than three weeks
away from demotion so that approvers aren't warned prematurely.
The belongs_to_creator macro was used to initialize the creator_id field
to the CurrentUser. This made tests complicated because it meant you had
to create and set the current user every time you wanted to create an
object, when lead to the current user being set over and over again. It
also meant you had to constantly be aware of what the CurrentUser was in
many different contexts, which was often confusing. Setting creators
explicitly simplifies everything greatly.
* Change minimum activity threshold to 30 approvals in 45 days.
* Exclude mods from losing privileges, not just admins.
* Fix undeletions of old posts not counting towards the activity threshold.
Sends automated dmails from `Danbooru.config.system_user`, rather than
whichever user is performing the action happens to be (usually User.admins.first).
Also adds a notice in the view that the dmail was automated.
