Support per-space display names and avatars

A proper solution for [per-room](https://github.com/vector-im/element-meta/issues/499)/per-space display names is currently blocked on https://github.com/matrix-org/matrix-spec/issues/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.

Start by reading the linked Element Meta per-room issue and matrix-spec issue 103, since the request says implementation is blocked there. Review the proposed space context-menu and room-list identity behavior in the issue description; the feature is done only when per-space display names and avatars can be set without global profile changes overriding them.

Written by the indexing model from the issue text.