Grammar and disambiguation of display names (SPEC-392)

Nobody has claimed this yet.

Both users and rooms can have display names. I **think** we can apply the same rules to both users and rooms, so I am covering both under this issue.

The emphasis here is on human-readability - they are not really meant for machine interpretation (modulo highlighting of mentions). Display names must therefore support the full gamut of unicode: non-bmp characters, zalgo, RTL scripts, etc.

However, things are complicated by the desire to disambiguate users. Currently, if we have two "Matthew"s in a room, we disambiguate them by also showing the user id. So the question is: how do we determine if we need to disambiguate users, and can we design the grammar of them to make this easier. For instance: do we allow empty displaynames, or those containing only whitespace? Do we allow names to start or end with whitespace, or contain sequences of whitespace characters? Do we mandate a certain sort of Unicode normalisation?

(Imported from https://matrix.org/jira/browse/SPEC-392)

(Reported by @richvdh)

Start with the Matrix specification's existing display-name and mention rules, then review the imported SPEC-392 discussion linked in the issue. Define the agreed grammar, whitespace and Unicode-normalisation requirements for user and room names, including how disambiguation should work, and document the resulting rules.

Written by the indexing model from the issue text.