Extend status field to include Horizon species

[DavidRoy]

Need to include 'Horizon' species on UKOT lists. Species not yet on territory but predicted to potentially arrive. This is best handled by extending the 'Establishment status' field as these species are additional to the existing categories.

By default, the horizon species should not be shown on current reporting pages

Will raise separate issues for how Horizon species are shown or not on other pages.

[johnvanbreda]

1. Add a new "Horizon" term to the options available for Establishment status.
2. Modify the views currently used to load species data (old and new browser styles) so that horizon taxa are excluded.

This sounds like a few hours work at most.