Fix #6856
Fix #6951

* Allow moderators to disable/enable login

* Instead of rejecting login, show forbidden error when login disabled

Avoid confusion because when login is rejected, the message is that
the account is not activated, which is wrong.

* Fix tests

* Admin: Show unconfirmed email address on account page

* Admin: Allow staff to change user email addresses

* ActionLog: On change_email, log current email address and new unconfirmed email address

* 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