Support multiple values for term_id and term_value_id in observations search API

[kueda]

Currently, I can't perform a search like "observations of male flowers" or "observations of dead larvae" even though I suspect it's technically possible, so I'd like to support this with query params like term_id=1,2&term_value_id=3,4 which would mean "show observations that have annotations of attr 1 and 2 and values 3 and 4"

If this is not technically possible, maybe this issue can serve as a place to write down why that is and how we might change the index to support that.