communities: module externalisation to new package
- INCOMPATIBLE Removes 'communities' module that has been externalised to separate Python package called 'invenio_communities'. Migration can be done by running pip install invenio_communities and adding 'invenio_communites' to PACKAGES. (closes #3008)
Signed-off-by: Tibor Simko <tibor.simko@cern.ch>