Cleanup/GitHub onlogin hook

[jykae]

Closes #1126

Proposed changes

• add Github npm package
• import & use to fix exception
• remove old admin role call from onLogin hook, handled by the package, https://github.com/apinf/meteor-first-user-admin/blob/master/server/accounts.js

[jykae]

onLogin hook seemed to be important anyway, added npm package, used import to get "Github not defined" exception away.

[frenchbread]

👍