Bug 35657: Add support for assignee_id to ticket_updates endpoint

Bug 35657: Add support for assignee_id to ticket_updates endpoint

This patch adds support for cross-synced ticket.assignee_id updates.

The API allows you to set assignee directly on a ticket or via a
ticket_update. In both cases we store a ticket_update with the fine
details of when and who set the assigee.

Signed-off-by: Paul Derscheid <paulderscheid@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>