* Add customizable user roles

* Various fixes and improvements

* Add migration for old settings and fix tootctl role management

- `GET /api/v1/admin/domain_allows` lists allowed domains
- `GET /api/v1/admin/domain_allows/:id` shows one by ID
- `DELETE /api/v1/admin/domain_allows/:id` deletes a given domain from the list
  of allowed domains
- `POST /api/v1/admin/domain_allows` to allow a new domain:
  if that domain is already allowed, the existing DomainAllow will be returned

* Add moderator role and add pundit policies for admin actions

* Add rake task for turning user into mod and revoking it again

* Fix handling of unauthorized exception

* Deliver new report e-mails to staff, not just admins

* Add promote/demote to admin UI, hide some actions conditionally

* Fix unused i18n