Remove omniauth gem

[nciemniak]

Remove omniauth gem and related code - no longer using omniauth.

For reference, here are a few of the commits where the omniauth functionality was initially added in:

• Add support for facebook logins
• Add Google+ login
• Move authorizations to separate model

Remaining TO DO:

• remove authorizations table and references to it

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Review checklist

• [ ] First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• [ ] If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• [ ] If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• [ ] Are all the changes covered by tests? Think about any possible edge cases that might be left untested.