Option to hide display name

### Your use case

#### What would you like to do?
I'd like to be able to hide display names, viewing only the full user URI, in both nick lists, `@`-references (including completion), and all events.

#### Why would you like to do it?
There are a few reasons, which I will iterate below.

- In many public channels, there are often users who set their display name to acryllic characters, emojis, long strings, or other character sets I'll henceforth refer to as "obtrusive".
- I think it is important to be able to know who is sending an event, and being mutable, the display name does not offer this.

#### How would you like to achieve it?

- A new setting phrased as `"{Always,Only} show {full,real} usernames"`, or `"Hide all display names"` that defaults to "off" in order to maintain the existing experience for all Element users
- If the setting is disabled (set to "off"), there should be no change from the current behavior of always showing display names.
- If the setting is enabled (set to "on"), always show the full username in any component that currently shows display names.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating the settings UI and the components that render display names in the Element clients, then trace nick lists, @-references and events. Done means an opt-in setting consistently shows full user URIs in all listed contexts while leaving the default behavior unchanged.

Written by the indexing model from the issue text.