Add event filters

A simple working site typeahead filter for /events.
Could use visual tweaks. C.f. https://github.com/swcarpentry/admintool/issues/13
In my opinion, if the `table_obj` or the `field_name` are wrong, the application should fail/raise an error rather than return an empty array. I.e. it is an exceptional case and should be handled as such (and SQLAlchemy will gladly complain about it).

Or am I misunderstanding why the error is captured here?