Sign in with GitHub to access app

[ameliejyc]

As a selection panel member I can login with my GitHub account to access the app. As an applicant I can login with my GitHub account to access the app.

Acceptance criteria:

• [x] Sign in with GitHub button to log into the app

[BartBucknill]

I was a wondering how easy it would be to get the username of the signed in user from github oauth as I couldn't see it in the docs. I found this stackoverflow question which seems to provide the answer, basically making a call the github api with the access token from the login in order to get the username.

[dangerdak]

This is dependent on #10 - it isn't necessary unless that's done, as only the meetup data needs to be kept private, the rest is publicly available anyway.

[dangerdak]

Something to decide on: sessions vs authorization token