fix keyboard navigation over disabled dates

Using keyboard arrows you can select dates outside the start/end date, or dates that fall into disabled week days.

There is an issue regarding keyboard navigation behavior
https://github.com/eternicode/bootstrap-datepicker/issues/792

Also there is a fix for selecting dates outside start-end, but doesn't fix disabled week days
https://github.com/CubaSAN/bootstrap-datepicker/commit/8f823a58d9410db1b482870808815979195e04f1