Add OAuth authentication

Screenshots/screencasts of the pull request introduced functionality.

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While reviewing pull-request (especially when it's your pull-request), please make sure that:

you understand what problem is solved by PR and how is it solved
new tests are in place, no redundant tests
DB schema changes reflect new migrations
newly introduced DB fields have indexes and constraints
there are no missed files (migrations, view templates)
required ENV variables added and described in .env.example and added to Heroku
associated Heroku review app works correctly with introduced changes

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fs / rails-base-graphql-api