Spec writing: put common values like senders, room IDs, aliases, etc in a shared location

At the moment each example is responsible for trying to be consistent with all its `$ref`s, which is a bit annoying. It'd be nice to be able to say `%(roomId)` or similar and have the spec generator automatically populate that field with a known value, allowing for consistency.

No files, tests, or entry points are named. Start by locating the spec generator and the examples that duplicate `$ref` values, then determine how shared values should be represented and populated; done means generated specifications consistently reuse values such as senders, room IDs, and aliases.

Written by the indexing model from the issue text.