HTMLMultiSelectField: Add 'dropdown' option for 'mw-chosen' behavior and document

Previously, you could pass 'cssclass' => 'mw-chosen' in the form
descriptor for a 'multiselect' field, and it'd be automatically
converted to a text field with a dropdown allowing values to be
selected. This is not very intuitive (unless you know what the Chosen
library is) and was not documented anywhere except for release notes.

The new recommended and documented way to achieve this is by passing
'dropdown' => true. Old way is supported for backwards compatibility.

Also, add the 'jquery.chosen' module to the page server-side.

Change-Id: I3a025e1c3c7571e930a35e020d73d558fdc433d0