Ability to override the display name for users & rooms that get served to you (SPEC-427)

Submitted by @​matthew:matrix.org
This could be done by account_data, both per-room (for overriding m.room.name and membership names within rooms) and globally (for overriding user displaynames globally)

(Imported from https://matrix.org/jira/browse/SPEC-427)

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Edit (richvdh 2020/08/11): I think this feature is asking for the ability to set local displaynames for rooms and other users, that are not shared with other users. For example, I might want to set a displayname of "Mum" for my mum, while her public displayname remains unchanged.

Review the account_data proposal and the 2020 edit describing local display names for rooms and users, then compare them with the imported SPEC-427 request. Define the protocol behavior and scope for per-room and global overrides, including what remains private; done means the specification fully documents the resulting behavior and edge cases.

Written by the indexing model from the issue text.