Consider unifying collapsible "dropdown" functionality

[jernestmyers]

In looking over options for a "dropdown" icon, I noticed that we handle this functionality in various ways.

In genomics records pages, we leverage WDKClient's CollapsibleSection component. In terms of icon, it uses Font Awesome's fa-caret-down and its related fa-caret-right. It looks as follows:

In web-eda's variable selection buttons, we'll see two different dropdown implementations in the popover: one for the Featured variables and another to expand/collapse the variables. Note the difference between the icons. The expand/collapse toggle for Featured variables uses the native HTML elements details and summary and thus uses the browser's default icon for such elements. On the other hand, the expand/collapse functionality in the variable tree derives from CheckboxTreeNode and uses the same icons as the CollapsibleSection. (Note also that the button itself uses the same icon.)

In the short term, I suspect we could leverage CollapsibleSection for the "Featured variables" to unify the icons in the dropdown menu. In the long term, we may want to create a version of a CollapsibleSection component in CoreUI that could meet our use cases described above.