Improve API for room tagging

Currently it's done on a per-room basis. Maybe we can promote the tag to be it's own object and add rooms to it?

Review the Matrix specification's current per-room tagging API and compare it with the proposed model of tags as objects that can contain rooms. The issue does not define the object shape, compatibility requirements, or acceptance criteria, so those design questions must be resolved before implementation can be considered done.

Written by the indexing model from the issue text.