Federation: Triggering remove activities on m2m_relation.clear()

Remove activities currently only triggered on post_remove.. they should be triggered on post_clear, too!

changed milestone to %0.7

mentioned in issue management/core-team#3

mentioned in merge request !134 (merged)

added 1h of time spent at 2020-06-04

closed via merge request !134 (merged)

mentioned in commit 29eca17b