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/javascript/styles/mastodon/admin.scss

@@ -448,3 +448,19 @@
     color: $success-green;
   }
 }
+
+.name-tag {
+  display: flex;
+  align-items: center;
+
+  .avatar {
+    display: block;
+    margin: 0;
+    margin-right: 5px;
+    border-radius: 50%;
+  }
+
+  .username {
+    font-weight: 500;
+  }
+}