* Don't set the inviter field for newly promoted users, or for Gold/Plat
upgrades.
* Clear the inviter field for paid Gold/Plat upgrades, and for users who
have a feedback or a modaction listing who invited them. This leaves
about 600 remaining users with an inviter field with no other record
of who invited them.
See #4750.
* Refactor to move upgrade logic from UserPromotion to UserUpgrade.
* Send the recipient and the purchaser of a gifted upgrade separate
dmail notifications.
Don't allow gifting Gold or Platinum upgrades to users above Platinum
level. Fixes an exploit where you could demote Builders and above by
gifting them an upgrade.
New rules for user promotions:
* Moderators can no longer promote other users to moderator level. Only
Admins can promote users to Mod level. Mods can only promote up to Builder level.
* Admins can no longer promote other users to Admin level. Only Owners
can promote users to Admin. Admins can only promote up to Mod level.
* Admins can no longer demote themselves or other admins.
These rules are being changed to account for the new Owner user level.
Also change it so that when a user upgrades their account, the promotion
is done by DanbooruBot. This means that the inviter and the mod action
will show DanbooruBot as the promoter instead of the user themselves.
Change message to "Your account has been updated". It's possible for a
user to both gain and lose permissions at the same time, so just say
their account has been updated to make it easier.
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.
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.
