R1:b038041dc600

Prevent duplicate account links from being created by swapping logins and then refreshing the link

Summary:
Fixes T6707. Users can currently do this:

- Log in to a service (like Facebook or Google) with account "A".
- Link their Phabricator account to that account.
- Log out of Facebook, log back in with account "B".
- Refresh the account link from {nav Settings > External Accounts}.

When they do this, we write a second account link (between their Phabricator account and account "B"). However…