Add fields support to query and tagsQuery

[daedalus28]

ES natively supports multi-field queries, so instead of creating multiple query/tagsQuery does in a contexture search and syncing them, we should just add support for fields which can be an array:

www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html#_multi_field

[daedalus28]

This would be an easy change, and might set a precedent for a cleaner virtual fields API - we'd just have fields take precedence over field (which might be a better model than dbField or realField). For query/filters that es doesn't natively support fields on, we could make utility that just runs the filter function for each field and ors them together (should in es parlance).

Either way, without an actual use case, this should definitely wait for a larger virtual field solution. @dubiousdavid FYI