Allow to script assigning a use case to packages

[jsrain]

When assigning the same use case implementation to a set of packages, the web UI does not scale. It would be much more convenient to allow doing that via a script

[crazyscientist]

Nobody is stopping you from writing a script for "batch" updating entities (i.e. send a myriad of HTTP requests to update single entities).

The API documentation can be found on the host at /api/docs.

Also, this issue seems to overlap with #184.