Add OAuth2 authentication (#147)

- Added Site settings > Authentication section
- Create/edit/delete your custom oauth2 configurations
- Login or signup with oauth2

config/routes.rb

@@ -26,6 +26,9 @@ Rails.application.routes.draw do
     
     resources :tenants, only: [:show, :update]
     resources :users, only: [:index, :update]
+    resources :o_auths, only: [:index, :create, :update, :destroy]
+    get '/o_auths/:id/start', to: 'o_auths#start', as: :o_auth_start
+    get '/o_auths/:id/callback', to: 'o_auths#callback', as: :o_auth_callback
   
     resources :posts, only: [:index, :create, :show, :update, :destroy] do
       resource :follows, only: [:create, :destroy]
@@ -50,6 +53,7 @@ Rails.application.routes.draw do
       get 'post_statuses'
       get 'roadmap'
       get 'users'
+      get 'authentication'
     end
   end
 end