[bugfix] prevent devise email uniqueness validation

users with different providers should be able to use the same email address. here is the line that we're overriding in devise:
https://github.com/plataformatec/devise/blob/83213569dd77551bb5cac493e2767457970905bc/test/rails_app/lib/shared_admin.rb#L12

app/models/devise_token_auth/concerns/user.rb

@@ -56,6 +56,10 @@ def email_changed?
       false
     end
 
+    def will_save_change_to_email?
+      false
+    end
+
     # override devise method to include additional info as opts hash
     def send_confirmation_instructions(opts=nil)
       unless @raw_confirmation_token