Calendar App got a new Maintainer

After the announcement at the mailinglist that the calendar app is unmaintained:

a new maintainer was found and the development of the ownCloud calendar is ongoing again:

Just to be clear on what is going on here from an owncloud perspective:
- @felixheidecke will maintain calendar
- in a very first step we will sync up the repos
- after that we will continue to develop this app
- nc is welcome to grab changes
- we will continue to grab commits from nextcloud/calendar as well