#32 taxon search should suppress unused taxa

[Mark Dominus]

BP says:

> -- Let's say that I go to the Taxa tab, select Text search, enter "Thermoplasma volcanium", and check the Taxon checkbox. The result is a list with two rows: one "Thermoplasma volcanium" and the other "Thermoplasma volcanium GSS1". Let's say I discard the GSS1 name. Then I click Studies and discover that there are no studies. I think it would be better for TreeBASE to only return taxa that are linked to studies -- that way people won't waste time clicking over to the studies tab when there is actually nothing to be found.