Enable/disable individual replication and snapshot tasks

- New functionality to enable/disable individual snapshot and replication tasks.
- Separated out Snapshot frequency from the 'How' field on Snapshot task Data Grid - this was to assist snapshot management as it was hard to see frequency when included in combined field with dates and times

This is a revised pull request following appreciated advice.  Schema migration script now created via manage.py schemamigration storage--auto.  I could not identify any other changes that would be needed to include in auto upgrade scripts.  pls advise if I have overlooked.

Jon