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/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/1971
* 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. Also, this was rebased several times, so cherry picking individual commits might result in buggy states.