Add GitHub authentication

We should allow users to integrate their GitHub account into their current account.

Some quick thoughts around this:

- We need to be able to associate most records (like `Comment` and `Task`) to a `User` that created them
- A `User` could be created by signing up with email, but could also be created via webhooks by some action on GitHub (or some other service)
- We can’t assume a user has an email associated with their account (GitHub may not provide this)
- Emails should be broken off as separate identity providers
- A GitHub account is also an identity provider
- When a user was created due to an external identity provider (claimed vs unclaimed), that user can be merged
- Merging will copy all relevant data from that user to the new user
- We probably need to punt on a user signing up / signing in with GitHub – they can only connect an account later – otherwise we could have a scenario where someone _intends_ to connect a Code Corps account to a GitHub account, but instead creates two separate accounts (one via email, one via GitHub) that are now disconnected. This would mean we'd have to some complex two-step verification process for a merge, which is beyond the scope of MVP here.