add rfc2307bis.schema into mixed groups plugin

Descriptive title for this enhancement

add rfc2307bis.schema into mixed groups plugin where it belongs and remove if from core

Actual behavior

schema is incorrectly in core

Benefits

have the schema where its needed

Possible Drawbacks

none

Applicable Issues

changed due date to May 15, 2020

changed milestone to %FusionDirectory 1.4

added PJ1802-0188 plugin-mixedgroups labels

added packaging label

mentioned in commit a7304962

removed due date

added Changed label

removed Changed label

added Added label

added 15m of time spent at 2021-01-28