Have 2FA affect only password logins

Description

The OAuth login providers we use on GitLab.com (Google, Twitter, GitHub and BitBucket) all support 2FA. It seems redundant to enforce 2FA for OAuth logins.

Proposal

Remove 2FA restrictions on https://gitlab.com. Make them affect only username/password logins.

Links / references

User idea:
https://twitter.com/tbroyer/status/819486296561381376