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/models/invite.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: invites
+#
+#  id         :integer          not null, primary key
+#  user_id    :integer
+#  code       :string           default(""), not null
+#  expires_at :datetime
+#  max_uses   :integer
+#  uses       :integer          default(0), not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class Invite < ApplicationRecord
+  belongs_to :user, required: true
+  has_many :users, inverse_of: :invite
+
+  before_validation :set_code
+
+  attr_reader :expires_in
+
+  def expires_in=(interval)
+    self.expires_at = interval.to_i.seconds.from_now unless interval.blank?
+    @expires_in     = interval
+  end
+
+  def valid_for_use?
+    (max_uses.nil? || uses < max_uses) && (expires_at.nil? || expires_at >= Time.now.utc)
+  end
+
+  def expire!
+    touch(:expires_at)
+  end
+
+  private
+
+  def set_code
+    loop do
+      self.code = ([*('a'..'z'), *('A'..'Z'), *('0'..'9')] - %w(0 1 I l O)).sample(8).join
+      break if Invite.find_by(code: code).nil?
+    end
+  end
+end