Handle github_nickname with multiple emails

[FRFlor]

Github supports the same account holding multiple emails. The Growth Sessions App, however, assumes 1 email per account.

This causes problems when people log in in the app using a different email account than they had before, whenever that happens the growth sessions app generates an entire new account for the person.

I can see 2 possible solutions for this:

1. At the oauth callback, check if the github_nickname already exists, if it does, log that person in as that account. (Probably the easiest and quickest approach)

2. Make growth session users be able to support multiple emails.