Bootstrap datepicker should support more formats

[Ghada El-Zoghbi]

Mahara: 19.04.0
OS: Linux 16.04
DB: Postgres
Browser: FF and Chrome (and others)

The bootstrap date picker supports the following formats:

* https://bootstrap-datepicker.readthedocs.io/en/latest/options.html#format

The date format, combination of d, dd, D, DD, m, mm, M, MM, yy, yyyy.

d, dd: Numeric date, no leading zero and leading zero, respectively. Eg, 5, 05.
D, DD: Abbreviated and full weekday names, respectively. Eg, Mon, Monday.
m, mm: Numeric month, no leading zero and leading zero, respectively. Eg, 7, 07.
M, MM: Abbreviated and full month names, respectively. Eg, Jan, January
yy, yyyy: 2- and 4-digit years, respectively. Eg, 12, 2012.

The Mahara lib/pieforms/pieform/elements/calendar.php file is not supporting all these formats and is still supporting the jQuery UI calendar.

The functions:

* pieform_element_calendar_convert_dateformat() and
* pieform_element_calendar_convert_timeformat()

Need to be updated to support the new date picker.

For example, this date format should be valid:

$string['pieform_calendar_dateformat'] = '%%d/%%mm/%%Y';

Instead, when updating the view access date/times, we get the error:

* Use the format DD/MMm/YYYY

Please note... this is not the only format to test for. Please test for other valid formats.