Be able to connect other oAuth providers from your settings profile

[grena]

1. You register with GitHub (no public email), it creates account with github_id & github_token.
2. In you profile settings, you choose to link to your google account, so it fills in google_id & google_token.

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1. You register with GitHub (no public email), it creates account with github_id & github_token.
2. You logout and register with Google.
3. In you profile settings, you choose to link to your github account, so it fills in github_id & github_token and remove other account with this github_id.