Support other not core MFEs

[igobranco]

The function iter_mfes_per_service has a problem that didn't support other not core MFEs.
To fix this just a line needs to be changed.

This line should be replaced

tutor-contrib-mfe-extensions/tutormfe_extensions/plugin.py

Line 79 in 2dbce5e

lms_mfes = set(CORE_MFE_APPS) - cms_mfes

by:

lms_mfes = set(active_mfes) - cms_mfes

So other MFEs could be supported, like ecommerce MFEs, or other custom MFEs.