[REQUEST] search geohash tags by prefix #265

A typical geohash filter will look for events in a given bounding box, which can be described by a list of geohash prefixes.
Following the discussion here, I think it would be useful if specifying a "*" at the end of a geohash will mean a search for all events with a geohash tag that contains the requested prefix.

I think the change should be somewhere here, but I'm not familiar with Knex. If filterName[1] is "g" and criterion.slice(-1) is "*", then instead of checking that event_tags contain a pair of [filterName[1], criterion], we should check that event_tags contain a pair with a first element of "g" and a second element that starts with criterion.slice(0, -1).

A possible alternative is to assume that every geohash filter that has less than 9 characters is actually a "prefix filter", even if it doesn't end with "*".

Another crazy option is to index every geohash value with all of its prefixes (i.e. for and event containing ["g","dqcjqcpew"], also add to the database ["g","dqcjqcpe"],["g","dqcjqcp"],["g","dqcjqc"],["g","dqcjq"],["g","dqcj"],["g","dqc"],["g","dq"],["g","d"] - this is crazy but will make the queries simpler.

Thanks!