Handle multiple accounts on GitHub errors

[db0company]

If someone tries to connect a GitHub account to another account, it shows the error: "AuthAlreadyAssociated" This github account is already in use.. It should display a clean error.

If someone uses an email address to create an account but this email is associated with a github account, when the user tries to connect to github, it should automatically link the 2 accounts into one (ie not create a user, just add a SocialAuth to the existing one).

[db0company]

To test the first case: go to a repository dashboard without being logged in (for example http://localhost:8000/Jucyio/Jucy/) then click on a heart to vote. It will show a pop-up to login, enter an email address (deby@jucy.io). Then click on your email address then "Your repositories". It will redirect you to /_oauth/login/github and will show this error.