Add moderator role and add pundit policies for admin actions (#5635)

* 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

config/locales/en.yml

@@ -62,6 +62,7 @@ en:
       by_domain: Domain
       confirm: Confirm
       confirmed: Confirmed
+      demote: Demote
       disable: Disable
       disable_two_factor_authentication: Disable 2FA
       disabled: Disabled
@@ -101,6 +102,7 @@ en:
       outbox_url: Outbox URL
       perform_full_suspension: Perform full suspension
       profile_url: Profile URL
+      promote: Promote
       protocol: Protocol
       public: Public
       push_subscription_expires: PuSH subscription expires
@@ -108,6 +110,11 @@ en:
       reset: Reset
       reset_password: Reset password
       resubscribe: Resubscribe
+      role: Permissions
+      roles:
+        admin: Administrator
+        moderator: Moderator
+        user: User
       salmon_url: Salmon URL
       search: Search
       shared_inbox_url: Shared Inbox URL