obs/tab: Enable searching/filtering on establishment means

[michaelpirrello]

For example, enable the ability to use ",obs anthophila from PA with means native"

[synrg]

Alternatively, ,obs anthophila from pa with native. There is no separate means parameter in the observations search API:

https://api.inaturalist.org/v1/docs/#!/Observations/get_observations

It just has native=true, native=false, introduced=true, introduced=false. If we stick closely to this model, we could have: ,obs anthophila from pa with native, with not native, with introduced, with not introduced, and with not native, not introduced (which is the only way via the API to find the "establishment means unknown" category, i.e. the taxon is listed, but set to "unknown").

Unfortunately, the API doesn't provide any way to find observations that are not listed. But that's a fourth possible category. As we discussed on #coding on Dronefly Discord server today, that can't be helped. It's up to the iNat developers to support that, if they feel it's a useful thing to support.