User Directory leaks Per-room Nicknames and Avatars

This issue has been migrated from [#5677](https://github.com/matrix-org/synapse/issues/5677).

---

### Update: October 2021

This issue has been resolved for a homeserver's local users.

We still need to address leaking per-room nicknames and avatars for remote users. This is complicated as we do not have an easy, obvious way to retrieve or keep up-to-date the public profile metadata for remote users.

---

### Description

The User Directory leaks display names and avatars for a user that are sent in only one room.
For example, by manually crafting a `m.room.member` state event – or recently using the `/myroomnick` command in Riot/Web, even if the state event is sent in a private room.

### Steps to reproduce

- Using Riot/Web with account `@alice:example.org`, open a private chat (such as a direct chat with a close friend)
- Issue the command `/myroomnick Freddy`, which sends a `m.room.member` state event into only that room with a custom nickname.
- From another account, say `@bob:example.org`, open up the User Search
- Search for 'Freddy' or 'alice' — `@alice:example.org` will be listed with the name 'Freddy'
- Note: this assumes that alice is visible to bob in the user directory – i.e. alice is in a public room known to the homeserver AND/OR alice and bob share a private room together.
- (Note that Synapse's user_directory table also reflects the change)

#### Expected Behaviour

alice's original display name should be shown in the user search.

#### Implications

This has privacy implications – a nickname set in a private room with a close friend may be quite personal and perhaps embarrassing if seen by other users.

### Version information

- **Homeserver**: librepush.net

If not matrix.org:

- **Version**: 1.1.0+bionic1

not really relevant, I suspect:

- **Install method**: Debian packages

- **Platform**: Ubuntu 18.04 in an LXC container on NixOS