Store blank MAC addresses as NULL
Improve dynamic interface MAC–Range uniqueness validation efficiency
Improve DynamicInterface.__str__ and __repr__
Clean migrated objects (including dynamic interfaces)
Print error message when migrated dynamic interface is invalid
Miscellaneous cleanup
Work around Django bug

If you write a custom model field whose get_prep_value, get_prep_lookup,
get_db_prep_value, or get_db_prep_lookup transforms a non-None value
into None, it generates '= NULL' instead of 'IS NULL' like it's supposed
to. That's because it determines the query type before it prepares the
field's lookup value (the order should be reversed, but patching Django
or getting a fix merged upstream would be much more difficult than just
disallowing a query value of '').
Rewrap long docstring; fix minor build bug
Don't make maintain_sb query at module import

The -q/--qlobber option of maintain_migrate requires that dns_migrate be
imported. Because get_delegated is called when dns_migrate is imported,
it's possible that the maintain sandbox database doesn't exist (which
raises an exception) or that the data in maintain_sb is out-of-date
(which causes results inconsistent with the intent of -q/--qlobber).
Merge remote-tracking branch 'origin/maintain_sb-fix' into improve_mac-range_uniqueness
Properly handle blank and invalid MAC addresses in Maintain
schema migration for #717 Enforce dynamic interface MAC–range uniqueness more efficiently