Fix: external providers with multiple emails

[erezrokah]

Fixes https://github.com/netlify/gotrue/issues/242 Related to https://github.com/netlify/netlify-cms/issues/3587

First attempt as I don't have a lot of go experience.

Instead of looking up the user just by a single email, iterate over all provider emails. When signups are enabled the primary email is preferred for registering new users.

Added more tests for GitHub, GitLab and Bitbucket (the last one required exposing the auth+api endpoints to be configured).

Would be happy to get some feedback and also planning on adding tests for processInvite (are there existing ones I didn't find?)

Update:

• Similar to Bitbucket, added tests for Google & Facebook
• Added tests for calling authorize with invite_token

[erquhart]

@erezrokah is this good to rebase and merge?

[erezrokah]

@erquhart, rebased and ready to merge.

[erquhart]

It looks like the pattern on other PR's is someone else approves but the opener merges.