previously selected values are carried forward. select2 behavior seems
to work to drop selected values if they no longer exist in the list of
available values.
refactor for testability
- MRB site type now actually filters stations down to MRB sites
- More refactoring
Added unit tests for filtering, actual UI events/wiring still not
tested.
test tweak
Merge pull request #282 from thongsav-usgs/NAWQAAN-376

Nawqaan 376