Allow CompleteGuidedSetup to post tasks in the #admins room

[yuwenmemon]

Coming from #51443 and the proposal outlined here.

Background

Right now, CompleteGuidedSetup posts setup tasks from the Concierge Account. The reportID it is posted into is provided by the parentReportID sent by the client when providing the optimistic task data.

Problem

Because the tasks are sent by the concierge account, we can't allow CompleteGuidedSetup to post tasks in the #admins room since the concierge is not part of that room (nor do we want it to be).

Solution

If the #admins room is provided as the parentReportID for a task, create the tasks from the guide in the #admins room who also sent the welcome message.

cc @MonilBhavsar @deetergp @c3024

[yuwenmemon]

This is complete