* feat: add assignee checking when updating flag
Prior to this, it was possible to update the assignee to any value (or
any user. This commit adds checking to allow only admins, global
moderators, or in the case of flagged posts, moderators.
Also some prep work was added for value checking `state`.
* feat: value checking `state` on flag update
The state should be one of the constants defined earlier in the file.