Remove special-casing of dates for slider formatting and fix datetimes

[mikmart]

Follow-up to #1119.

This PR removes the now-unnecessary special-casing from formatDate() as the mechanism from #1119 handles it directly.

This also fixes a regression introduced in #1119 for datetime columns, which was caused by me overlooking to handle datetime values when applying a renderer to format the slider range labels.

There's some duplicated logic in the JS side formatDate() helper and restore(). I thought about refactoring that, but it would be a slightly more involved operation without a big direct benefit, so I decided not to mess with it at this point.

I tested manually with this app:

DT::datatable(
  data.frame(
    date = Sys.Date() + 1:10,
    time = Sys.time() + 1:10 * 60 * 60 * 24
  ),
  filter = "top"
) |> DT::formatDate(1:2, "toLocaleString")

[yihui]

Excellent! Thanks!