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

config/routes.rb

@@ -22,6 +22,10 @@ Rails.application.routes.draw do
   get 'manifest', to: 'manifests#show', defaults: { format: 'json' }
   get 'intent', to: 'intents#show'
 
+  devise_scope :user do
+    get '/invite/:invite_code', to: 'auth/registrations#new', as: :public_invite
+  end
+
   devise_for :users, path: 'auth', controllers: {
     sessions:           'auth/sessions',
     registrations:      'auth/registrations',
@@ -99,6 +103,7 @@ Rails.application.routes.draw do
   resources :media,  only: [:show]
   resources :tags,   only: [:show]
   resources :emojis, only: [:show]
+  resources :invites, only: [:index, :create, :destroy]
 
   get '/media_proxy/:id/(*any)', to: 'media_proxy#show', as: :media_proxy
 
@@ -112,6 +117,7 @@ Rails.application.routes.draw do
     resources :email_domain_blocks, only: [:index, :new, :create, :destroy]
     resources :action_logs, only: [:index]
     resource :settings, only: [:edit, :update]
+    resources :invites, only: [:index, :create, :destroy]
 
     resources :instances, only: [:index] do
       collection do