Refactor Circuit Overseer Support

[BenShelton]

Currently we store CO details as a sort-of meta field on the week.

If we actually created a custom assignment for the service talk and allowed them to be added as a special kind of congregation member, it's going to avoid all the extra workarounds we need as well as make it easier to inherit the week for the CO visit.

Bear in mind it would be annoying to show the name for all weeks, so maybe a special exception for the AssigneeSelect is that it only shows their name on CO weeks.