On create/edit custom role page, show placeholder for base role dropdown and uncheck first item

On the create/edit custom role page, @svedova reported that the base role dropdown has the first item pre-checked when the dropdown itself shows nothing selected:

@ipelaez1 said that we should do the following:

1. Add Select role as the placeholder text.
2. Remove the first item as checked.

We likely have to switch the base role dropdown from a GlFormSelect to a GlCollapsibleListbox. We should also investigate whether the "first item as checked" behavior is OSX-specific, since it does not occur for Linux.