No entry in device_lists left when user rejects invite

This issue has been migrated from [#6163](https://github.com/matrix-org/synapse/issues/6163).

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In https://github.com/matrix-org/synapse/blob/master/synapse/handlers/sync.py#L1392 we only include a member in `newly_left_users` if their previous membership was `join`, but in the other branch of the `if`, we include them in `newly_joined_or_invited_users` if they're, well, joined or invited, so if a user goes from `invite` to `leave` (or `ban`) then we'll never send them in the `left` section.

NB. clients shouldn't be relying on this to work out who to encrypt to (and indeed the js-sdk does not) so this doesn't cause events to be encrypted to anyone they shouldn't be. It might just cause clients to continue tracking the device lists of some users when they don't need to.

Start in synapse/handlers/sync.py around line 1392 and trace how membership transitions populate newly_left_users and newly_joined_or_invited_users. Reproduce the invite-to-leave and invite-to-ban cases, then verify that the affected users appear in the left section so clients can stop tracking their device lists.

Written by the indexing model from the issue text.