Feature: add reference organisms toggle

[jernestmyers]

Depends on https://github.com/VEuPathDB/CoreUI/pull/144

Things of note in this iteration:

1. The width of the left panel container has been increased 25%
2. The size of the [Ref] string has been reduced slightly
3. The hover effect that bolds the checkbox node has been removed since it enlarges the element which can cause annoying wrapping
4. The "references only" toggle behaves like a search filter by a) expanding the tree, b) preserving checkboxes' indeterminate states, c) rendering the same tree links, and d) removing/disabling the expand/collapse toggles in the rendered tree
5. A tooltip reads "Show only reference organisms [Ref]"