Make it possible to auto-import existing users when the localpart matches in upstream OAuth 2.0 logins

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2089.

When there is a new upstream OAuth 2.0 link and the localpart imported matches an existing user, we have 4 choices:

• do nothing, and fail the registration/login (corresponds to allow_existing_users: false in Synapse)
• import the link if and only if there is no existing link for this user
• import the link and replace any existing link for this user
• import the link without overriding existing links for this user (corresponds to allow_existing_users: true in Synapse)