Upgrade assets to fullcalendar-4.3.1

[EverardB]

[yshmarov]

Looks great!

[AxelTheGerman]

@bokmann there have been several pull requests for newer versions from @EverardB any chance those could get merged in and released to rubygems?

I know many folks probably switched over to webpacker but I usually just need one or two dependencies so I prefer the old school asset pipeline over the complexity of webpack (even though much is hidden - but still there - by webpacker)

[madaarya]

No readme changed? seems the fullcalendar js path different than before