Switch datepicker CSS @import to a 'require' to fix datepicker regression

Previously, the datepicker showed with a transparent background
because the base jquery UI css wasn't loaded.

Since upgrading to Sass 5, apparently @importing the datepicker
CSS doesn't require the base jquery UI css to be loaded.

The datepicker CSS file imports the jquery UI CSS through a
sprockets 'require' statement, so I guess Sass 5 accidentally
broke that "@import -> require" pathway.

It's easy enough to just use a 'require' in this case instead,
which works perfectly well.