Improved mechanisms for discovering rooms (SYN-210)

This issue has been migrated from [#1260](https://github.com/matrix-org/synapse/issues/1260).

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[23:16] hmm anyone know if matrix allows people with similar interests to find each other in a conference room without actually knowing each other first?
[23:18] hmm, there's nothing explicitly to support it
[23:18] and our room listing mechanisms are pretty basic at the moment
[23:18] but it'd certainly be interesting to support specifying more metadata about rooms than their topic, name and alias
[23:19] to make that easier

Aside from this, we need APIs for searching & paginating the public room listing, and exposing for search engines (and possibly aggregating across multiple homeservers)

(Imported from https://matrix.org/jira/browse/SYN-210)

(Reported by @ara4n)