Migrate GPG plugin to selectManagement

GPG plugin is still using simpleSelectManagement, partly because of #5994 (closed)

See https://gitlab.fusiondirectory.org/fusiondirectory/fd/issues/5965#note_65002

Related to #5937 (closed)

added plugin-gpg label

mentioned in commit a788404b

mentioned in commit a98876d9

mentioned in issue fd#6045 (closed)

added PJ1802-0188 label

added 3h 30m of time spent at 2019-10-30

added To Be Tested label

assigned to @jswaelens and unassigned @MCMic

Tested and work correctly on my virtual machine

closed

added 1h of time spent at 2019-12-04

added Changed label and removed To Be Tested label