[Rooming] Multiple contexts for rooming

Attend should support pre, and post event rooming being separate lists, and having a select list of attendees that are roomed in a hotel, instead of assigning _everyone_ a room.

Start by locating the existing rooming flow and its attendee-assignment entry point, then trace how rooming data is stored and presented. Confirm how pre-event and post-event rooming are currently represented. Done means those contexts have separate attendee lists and hotel rooming can be limited to a selected set rather than everyone.

Written by the indexing model from the issue text.