Support EasyAuth 0.4.0

Initial effort to support EasyAuth 0.4.0 with `uid` as an array in the identities table.

I'm not convinced what I did with the db here is good, but is more similar to what was there before and adding a whole dummy application (like on EasyAuth-password) seemed too much.

I also got stuck when the `uid` retrieved from the OAuth service is not present in any identity. With the `overlap` and `first_or_initialize` the uid value was not being set on the new identity record, so that's why I added  `identities/oauth2/base.rb#18`. If there's a better way let me know and I'll change it.

To test this, you'd need to create the `easy_auth_oauth2_test` database manually and add the EasyAuth's GitHub repo after the `gemspec` call in the Gemfile.