Ship the OpenWebCalendar on flathub

[niccokunzmann]

I do not see an application that can just display a webcal link or ics file.
The Open Web Calendar could be shipped on flathub to provide this functionalilty!

See also:

• https://superuser.com/questions/1247054/firefox-standalone-web-app-on-desktop-similar-to-chrome-add-to-desktop