Bumped version for release of 2.1.20
Merge branch 'release/2.1.20' into develop
Bumped version for development after release.
Fixed some legacy groups where getMembers were inconsistent with isMember. Fixed senders and recipients delete().
Changed BennuGroupBridge group to have as expression just the externalId
New PersistentGroup implementations and conversion mechanism from the old group system
Removed group  translation from classes known to be deletable