Add force_login option to OAuth authorize page (#8655)

* Add force_login option to OAuth authorize page

For when a user needs to sign into an app from multiple accounts
on the same server

* When logging out from modal header, redirect back after re-login

app/views/layouts/modal.html.haml

@@ -8,7 +8,7 @@
       .name
         = t 'users.signed_in_as'
         %span.username @#{current_account.local_username_and_domain}
-      = link_to destroy_user_session_path, method: :delete, class: 'logout-link icon-button' do
+      = link_to destroy_user_session_path(continue: true), method: :delete, class: 'logout-link icon-button' do
         = fa_icon 'sign-out'
 
   .container-alt= yield