Support syncing fields across foreign keys

It should be possible to sync fields across foreign keys (reverse ones, even).

assigned to @nik

added part::backend source::customer type::feature labels

created merge request AlekSIS-App-LDAP!34 (closed) to address this issue

mentioned in merge request AlekSIS-App-LDAP!34 (closed)

This is actually a pure core feature, to fill syncable_fields correctly.

moved from AlekSIS-App-LDAP#21 (moved)

created merge request !396 (merged) to address this issue

mentioned in merge request !396 (merged)

closed with merge request !396 (merged)

mentioned in commit 3941d44f