[meta] Restore basic functionality.

Remaining:
#2346097: Use fullcalendar.permissions.yml
#2827930: "Undefined index: display" errors after changing view's format
#2827936: Bad URL in AJAX requests
#2827939: Error: Call to undefined method Drupal\Core\Access\AccessResultAllowed::cachePerRole()
#2827950: Call to undefined function l() in AJAX Update controller
#2828939: Restore AJAX functionality
#2853942: Libraries are not attached for plugins

Original post:
It has been a little time since this module was updated --- core is constantly changing.

This is just a parent issue for a bunch of narrowly focused issue digging through change records and restoring basic functionality.

I can see 2 issue that need solving ... and that was within the first 5 mins of looking so I expect more.

1) We are still using String::checkPlain - when SafeMarkup::checkPlain is the well established replacement

2) drush en fullcalendar identifies compatibility issues with Drupal\fullcalendar\Form\SettingsForm

3) The l() function has been removed from core.

4) submitForm method needs work.

Nice to have:

While not strictly essential to restoring basic functionality. From experience whenever I am working on these types of projects
with coupled javascript I have found converting all the legacy javascript to using strict CAN resolve lots of funnies ahead of time.