Cache list of active samples when the active study is changed in the plating interface

This should speed up the client-side matching process, as implemented in #585. This will be helpful when plating samples for huge datasets (e.g. AGP).

I imagine this should be pretty simple to implement (whenever the active study changes, update an `activeSamples` array or something that's stored as a property of the `PlateViewer` object or less ideally as a global variable).

Issue created based on [this comment](https://github.com/biocore/LabControl/pull/585#discussion_r322500725) by @AmandaBirmingham.