Remove jquery-ui; in a later commit we'll add it back via the gem

The intention is to stop including our own custom build of jquery-ui
but instead use the jquery-ui-rails gem, which works well with the
asset pipeline.  This commit should remove all traces of the old
download of jquery-ui.
Update to jquery-rails to a version without jquery-ui

The jquery-rails documentation for the latest version now recommends
using the jquery-ui-rails plugin instead.  To avoid the possible
confusion of having two copies of jquery-ui in different gems, update
jquery-rails to the version that doesn't contain jquery-ui.
Use the jquery-rails-ui gem, just including the modules we need

As far as I can tell, we only use the 'tabs' module in
admin and 'datepicker' on the user-facing part of the site.

The advantage of using this packaging of the gem is that
its assets are in the gem, which simplifies things greatly -
otherwise we'd end up doing something like rewrite the jquery-ui
CSS to SCSS, referencing the image assets via sass-rails helpers
or keep them in their expected paths in public or something.

(Thanks to Louise Crow for pointing out the problem of just
moving jquery-ui's image assets into the asset pipeline.)