Feature/copy projects

This PR implements most (NOT all) tickets of https://www.openproject.org/issues/1959. Relevant stories are:
* https://www.openproject.org/issues/1954
* https://www.openproject.org/issues/1955
* https://www.openproject.org/issues/1965
* https://www.openproject.org/issues/1967
* https://www.openproject.org/issues/1968
* https://www.openproject.org/issues/1969
* https://www.openproject.org/issues/1970
* https://www.openproject.org/issues/1972
* https://www.openproject.org/issues/1973
* https://www.openproject.org/issues/1974
* https://www.openproject.org/issues/1975
* https://www.openproject.org/issues/1976

This is a work in progress, so I'm not sure everything will work 100%. Stuff like copying work package categories with an assigned user WITHOUT copying members as well might break.