Custom Dropdown tab-compatibility

Before, custom dropdowns were confusing because they cannot be tabbed to like other inputs, but contain two links which can be. For two tab-presses, it appears the form is broken.

Now, users can tab to a dropdown, which opens it, and enables existing interactions. whether or not the dropdown is opened, users can also tab to the next input and the dropdown will close or stay closed.
Merge pull request #2963 from Shreve/master

Custom Dropdown tab-compatibility