Grammar for room aliases (SPEC-391)

Room aliases are intended to be
- human-readable
- unique

They have some structure: `#:`.

We need to specify a grammar for them, including which characters they may contain, and how they should be interpreted. Can they contain punctuation or whitespace characters? Should they be allowed to contain non-ascii characters, and if so, how should they be normalised? Should they be case-insensitive, and if so, in what locale?

(Imported from https://matrix.org/jira/browse/SPEC-391)

(Reported by @richvdh)

Start with the room-alias structure described in the issue and review the imported SPEC-391 context. Resolve the open questions about allowed characters, whitespace, non-ASCII normalization, and case handling, then document the grammar and interpretation rules in the Matrix specification.

Written by the indexing model from the issue text.