Add topic support

[bernhold]

New metadata on events: 'topics: ["online learning", "topic 2". "topic 3", ...]' will modify the metadata generated for BSSw events (currently static).

If not present, it should generate "online learning" for historical consistency. If present, only the listed topics should be used, none should be added (for transparency).

Might be nice to have a list of topics to compare against to avoid errors, though note that list can change at the whim of the BSSw Editorial Board.