FIX: allow storage of non unique rows in oauth2_user_infos

Certain DBs have duplicates already, if we want to ensure uniqueness here
we need to decide first how to clean up existing data and confirm all the
plugins expect this.

db/migrate/20191022155215_add_index_to_oauth2_user_info.rb

@@ -2,6 +2,6 @@
 
 class AddIndexToOauth2UserInfo < ActiveRecord::Migration[6.0]
   def change
-    add_index :oauth2_user_infos, [:user_id, :provider], unique: true
+    add_index :oauth2_user_infos, [:user_id, :provider]
   end
 end