We don't provide enough info in invites to unnamed rooms to name them correctly in clients.

This issue has been migrated from [#12835](https://github.com/matrix-org/synapse/issues/12835).

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If you:

* create a private room (e.g. a DM)
* invite someone to it
* their client will show an invite that says "Matthew invited you to chat", rather than showing the correct synthetic name of the room (e.g. "Matthew and Amandine and 5 others").

We should provide sufficient state in the invite event for the target client to correctly synthesise the right name.

If this needs a spec change, then i'd expect one to be filed alongside fixing this in synapse.

This is a major UX shortcoming - refiled from https://github.com/vector-im/element-meta/issues/379