Add consumable invites (#5814)

* Add consumable invites

* Add UI for generating invite codes

* Add tests

* Display max uses and expiration in invites table, delete invite

* Remove unused column and redundant validator

- Default follows not used, probably bad idea
- InviteCodeValidator is redundant because RegistrationsController
  checks invite code validity

* Add admin setting to disable invites

* Add admin UI for invites, configurable role for invite creation

- Admin UI that lists everyone's invites, always available
- Admin setting min_invite_role to control who can invite people
- Non-admin invite UI only visible if users are allowed to

* Do not remove invites from database, expire them instantly

app/views/admin/action_logs/_action_log.html.haml

@@ -1,7 +1,7 @@
 %li.log-entry
   .log-entry__header
     .log-entry__avatar
-      = image_tag action_log.account.avatar.url(:original), alt: '', width: 48, height: 48, class: 'avatar'
+      = image_tag action_log.account.avatar.url(:original), alt: '', width: 40, height: 40, class: 'avatar'
     .log-entry__content
       .log-entry__title
         = t("admin.action_logs.actions.#{action_log.action}_#{action_log.target_type.underscore}", name: content_tag(:span, action_log.account.username, class: 'username'), target: content_tag(:span, log_target(action_log), class: 'target')).html_safe