Make select2 widgets accessible

From https://civicrm.stackexchange.com/questions/17735/access-for-blind-users-to-civicrm/17752#17752

Civi makes use of the Select2 library in a number of places to generate certain pop-up lists. Basically, the library creates an edit-combo box (one where you have both a pre-defined list of options, but can also type in to narrow this list). While these elements are keyboard accessible, there are occasionally focus issues. As an example, choosing a date range filter on searches offers selections like, “previous year,” but also allows the user to choose a date range. When selecting the choose a date range option edit fields appear next to the dropdown allowing the user to enter a start and end date range. However, for a screen reader user, pressing TAB after selecting this option will focus on the next group of fields, bypassing the date range fields. Although they don’t use Select2, this behavior is also observed with combo boxes that dynamically populate other combo boxes (like on the import data screens).

Tech spec to come.