Link several login options to one account

**Is your feature request related to a problem? Please describe.**
I created my account with a Google account and that is now the only entry point.

**Describe the solution you'd like**
It would be good to be able to set a password and add a Github login or vice versa

The issue concerns the existing Google, GitHub, and password login entry points, but names no files or tests. Start by locating the authentication and account-identity flows for those entry points. Done means an account created through one option can securely add and use the other available login methods.

Written by the indexing model from the issue text.