Audience selector on "more" filter knowl should be a multi-selector

[poonen]

Katy Croff Bell's email made me realize that it would make more sense if one could select more than one "Audience" type when restricting a search on the "more" knowl. People interested in talks matching one audience will likely also be able to understand talks matching a broader audience too.

I have already adjusted the "Audience" knowl on the Edit series and Edit talk pages to reflect this.

[poonen]

Just to clarify, I am assuming that organizers are still going to choose just one audience type, but maybe we should rethink that as well.