sql: Add status field

Add a new field called status to reflect the status of a prefix, such as
'active', 'reserved' or 'quarantine'.

Part of #494.
backend: add status field

No extra handling really, just a straight mapping of the value from API
to database.

Part of #494.
pynipap: add status field

Part of #494.
CLI: add status field

Part of #494.
www: add status field

Part of #494.
tests: fix tests for new status field

Part of #494.