Linking and migrating address cores connected to multiple 3IDs

[maurerbot]

There will be occasions when the same account (be it blockchain, social, etc) is attempted to be linked to more than one 3ID core. This could be because:
A) The account is shared or
B) The user used said account to authenticate into 3ID before it was every linked and now needs to migrate it.

In either case the user should be made aware of the multi linking situation and be given some choices.

In either case the user should be prompted that the account is already linked to another 3ID and if that 3ID is in their control they can unlink it and come back to link it or connect it anyways is a "also known to" list in the 3ID (or a uni directional connection).

The "also known to" list maintained by a 3ID core can have similar functionality to a standard link but will not be able to be used a 3ID resolver as this would be difficult to manage.

cc @szkl @alfl