We need a better way of picking the user(s) to name an anonymous room for (SPEC-267)

When a room has no explicit name or canonical alias, we name it after the members of the room ("Alice and Bob", "Alice and 3 others", etc).

We would like this name to be stable and consistent across clients. We need an algorithm to pick the right users.

(Imported from https://matrix.org/jira/browse/SPEC-267)

(Reported by @richvdh)

Start by reviewing the requirements in SPEC-267 and the existing Matrix specification text for naming anonymous rooms. Define an algorithm that selects users consistently across clients, and document the rule and its expected naming behavior in the relevant specification section.

Written by the indexing model from the issue text.