[r=mpontillo][bug=][author=mpontillo] Support for unmanaged subnets.
Part 1: Begin adding support for unmanaged subnets.
* Adds a 'managed' field (default=True) to the Subnet model.
* Allows manipulation of the 'managed' field using the API.
(Updates the SubnetForm so that the new field can be used
during creates or updates.)
* Updates websocket handler tests to account for the new field.
* Updates test factory to account for the new field.
* Adds (and, for consistency, reformats) API documentation to
explain what the 'managed' flag does.
Part 2: Update IP allocation algorithm for unmanaged subnets.
* When choosing the next IP address to automatically allocate
from an unmanaged subnet, include *only* IP ranges of type
RESERVED in the set of possibilities.
* Update subnet tests to add scenarios for unmanaged networks.
* Drive-by fix to change legacy term 'comment' used in MAASIPSet to
be 'purpose' (for consistency).
Part 3: Add 'managed' flag to subnet details page.
Part 4: Exclude unmanaged subnets when generating DHCP configuration. Update UI text accordingly.