Support for room aliases

[thejhh]

• Repository implementations for aliases
• Add alias to the room creation
• Add alias API end points

We don't use aliases at the moment since aliases are global, hence not very useful for custom application use cases where naming scope should be application specific -- not global. (Aliases of other applications would overlap.)

However, it could be implemented since hghs would enable to run a standalone HS for any app instance. Then, alias would be easy way to uniquely name objects inside single application instance.

See also

• [TASK] [TBD HTP] Event m.canonical_alias #33