Pickadate format based on js format

[r-xyz]

Closes #237 by removing hard-coded date/time formatting in kiosks( 1, 2, 3) and mobile view (4, 5).

[r-xyz]

The new utilities:

• keep untouched all format which are the same in js and pickadate
• convert all supported formats which are different to the closest equivalent
• remove all unsupported formats.

It might not work as expected with very exotic format strings, but it should work in most cases (including default) while avoiding adding an extra setting.

Yet I can add the extra setting as an override, if you prefer.

[rptmat57]

I think I'd like to have the override options in settings.py, just to be safe

[r-xyz]

Fixed previous approach (I noticed it was not covering all conversions).
Added optional settings in settings.py; if not set, conversion will be attempted.

[rptmat57]

looks good, thanks!
would you mind renaming the variables to pickadate_date_format and pickadate_time_format?