Add a "user" property to the IndividualAggregator

This contains the Individual representing the user who owns all the relevant
accounts in each backend. This also changes the linking code so that Personas
with is_user == true are always linked, and only ever linked to other Personas
with is_user == true (i.e. Personas are always linked to Personas with like
is_user values). This ensures that there's only ever one Individual with
is_user == true. Helps: bgo#627402