Ability to toggle snapshot and replication scheduled on/off

This change is mainly to enable individual snapshot and replication schedules to be toggled active or inactive.  State is displayed in the 'periodic Snapshot Tasks' and 'ZFS Replication' tabs.  I note ticket #2004 was a request for maintenance mode which was closed as 'already implemented' but I can not see that it is.

Also some minor label changes

NOTE
This change introduces a new data field on replication and snapshot tables.  Based on template from previous migration scripts I have created 0039_auto_add_fields_replication-repl_active_storage_task_task_active.py  HOWEVER I do not know where/how this is called in upgrade functions(s)