Add property 'contributionTypes' to 'properties' parameter

[FabiKo117]

Currently, the @contributionType field gets added to the response features of the /contributions/{geometryType} endpoints if the properties=metadata parameter is given in the request. However, as it is not directly related to the other fields that get added via setting this parameter, it would be better to have it as a distinct value of the properties parameter, so that it gets added via setting properties=contributionTypes (or alternatively contribution types). This property should only be allowed for the /contributions/{geometryType} endpoint.

[tyrasd]

I changed this to milestone 1.5 since the current solution is not very good: It intermingles two different output options: OSM metadata and the contribution type. For backwards compatibility we probably have to stick with the side effect of metadata enabling the contribution type output. I 2.0 we can think about deprecating it, so that metadata only returns metadata :)