Remove support for users entering unseparated mmddyy and mmddyyyy dates

[tobiasnteireho]

mmddyy and mmddyyyy is not a clear format and presents validation problems. Since datepicker is implemented and explicit format is specified as mm/dd/yyyy support for unseparated and mmddyy and mmddyyyy is unnecessary and costly to support.

related to #256

[JessicaFrank]

the mm/dd/yyyy format is suggested, but not required for the end user. Can we enforce the format if we remove the validation?

[tobiasnteireho]

mm/dd/yyyy is enforced and required while disallowing mmddyy and mmddyyyy by #262

fixed by #262