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

spec/models/invite_spec.rb

@@ -0,0 +1,30 @@
+require 'rails_helper'
+
+RSpec.describe Invite, type: :model do
+  describe '#valid_for_use?' do
+    it 'returns true when there are no limitations' do
+      invite = Invite.new(max_uses: nil, expires_at: nil)
+      expect(invite.valid_for_use?).to be true
+    end
+
+    it 'returns true when not expired' do
+      invite = Invite.new(max_uses: nil, expires_at: 1.hour.from_now)
+      expect(invite.valid_for_use?).to be true
+    end
+
+    it 'returns false when expired' do
+      invite = Invite.new(max_uses: nil, expires_at: 1.hour.ago)
+      expect(invite.valid_for_use?).to be false
+    end
+
+    it 'returns true when uses still available' do
+      invite = Invite.new(max_uses: 250, uses: 249, expires_at: nil)
+      expect(invite.valid_for_use?).to be true
+    end
+
+    it 'returns false when maximum uses reached' do
+      invite = Invite.new(max_uses: 250, uses: 250, expires_at: nil)
+      expect(invite.valid_for_use?).to be false
+    end
+  end
+end