Add pseudo field for “Profile on primary site” link

Pseudo fields are more useful for getting content on a page than hook_user_view() nowadays. They will work better with more display layers, like Display Suite, Panelizer, and basic re-arrangability with core-only.

The code in bakery_user_view() needs an implementation of hook_field_extra_fields() to get it in the field UI.

I expect $account->content['summary']['master_profile'] will need to change to something like $account->content['bakery_primary_profile_link'] — not a nested array, and better namespaced.