enhance /users/count endpoint with contributionType parameter

[tyrasd]

Use Case Description

To analyze user activity with respect to different contribution patterns, it could be useful to narrow down user counts by contribution type, like already implemented for the contribution count endpoints in #198.

Request Description

The /users/count endpoint would return the number of users which performed at least one creation/deletion/*modification operation during the respective time intervals of the query. These numbers would be typically only slightly lower than the "unfiltered" total user count.

[bonaparten]

Should it be implemented only for the endpoint /users/count or are there some use cases also for the other resources (/users/{resource})?

[tyrasd]

I guess it would make sense for all /users endpoints, i.e.:

/count
/count/groupBy/(boundary, key, tag or type)
/count/density
/count/density/groupBy/(boundary, tag or type)