Add suggestions for which users to invite to chat
Fixes https://github.com/vector-im/riot-web/issues/11198
Note this doesn't implement the entire algorithm in 11198 because it feels too complicated at this stage. Instead, the idea is to review the suggestions closer to when the whole dialog is complete and fix them then: https://github.com/vector-im/riot-web/issues/11769
Algorithm for picking members is largely based on https://github.com/matrix-org/matrix-react-sdk/commit/db5218e19a26b650d4af37de81f516c68ab7e9b8
