Support per-space display names and avatars

[robintown]

A proper solution for per-room/per-space display names is currently blocked on matrix-org/matrix-spec#103, so I'm just leaving this here to document the feature request.

Is your suggestion related to a problem? Please describe.

People frequently wish to have their display name appear differently to different communities, and this is a feature that many expect when coming from messengers such as Discord. While this is technically possible with /myroomnick, users should be able to set their display name for an entire space in a single step, and this setting should not be overridden when they change their global profile.

Describe the solution you'd like.

I would like there to be an option in the right-click menu of spaces to change your display name in that space, with a little hint under the space name to help the user discover that they can have different identities in different spaces:

When the user has that space selected, any changes should be reflected up at the top of the room list, where the display name and the name of the current space are listed side by side.

Describe any alternatives you've considered.

The 'change your name' option could be put in the user menu at the top of the room list, which might make more sense since it's right next to your display name, however this feels a bit inappropriate since that menu is for global application options.

[t3chguy]

How would this work given that a room may be in multiple spaces at once? Which name/identity would be used in such a scenario

[robintown]

In my opinion it would be fine to arbitrarily give preference to either the new or the existing custom display name for that room, since the user could then override it with room-specific controls (#2458) if it mattered to them.

[HarHarLinks]

In case the other space/room already has a name set, element could also ask

[robintown]

I would also add that it could be nice to go one step further and also have per-space avatars, in which case the option could read "Change your profile".

[robintown]

Spec change is MSC3189

[tmonz]

that would be great. just had a couple of issues/delays, because some entities know me in one space under another nick, and then blocked my dm because they didn't know me by my 'standard' account name.

[audunmb]

I use Element both for work and private communication, and really need this. I guess as matrix.org/Element picks up more use, this would be even more of an issue.