Option to always disambiguate matrix users

Currently, matrix IDs are only shown when the display name is ambiguous in a room. However, I often chat with the same people across time and several rooms and would like to be able to easily tell if I'm actually am talking to the same person. Therefore, it would be nice to have a better way to uniquely identify a matrix user at a glance. vector-im/element-meta#1201 addresses this to an extent but requires manual action.

One simple solution would be to have an option to always show matrix IDs: "Display Name (small-matrix-id)".
To shorten IDs, the homeserver could be removed if it matches the main address for the room or, alternatively, the current user's home server (although that punishes users who run their own homeservers).

Start by reading this issue and vector-im/element-meta#1201 to understand the existing approach to identifying Matrix users. Define the user-facing option and its ID-shortening behavior, then verify that the display is unambiguous across rooms and that the option works consistently in the affected Element clients.

Written by the indexing model from the issue text.