Change CSS class names to avoid conflict with core calendar module

[mattdy]

Use of the today and tomorrow CSS class names conflicts with the calendar module included with the core MagicMirror software (see https://github.com/MichMich/MagicMirror/blob/master/modules/default/calendar/calendar.js#L208) since release 2.22.0 (https://github.com/MichMich/MagicMirror/commit/0300ce05d5e7af4584f73115aa590530af0a7655)

This PR changes the class names in use within this module to something less generic to deconflict.