Add status field

Add the new status field as per #494. There are three statuses; 'active', 'reserved' and 'quarantine' to reflect the status of a prefix. One might think that 'active' should have a counterpart as in 'inactive' or similar, but if a prefix isn't used it should probably be removed or be set in 'reserved' state.

On upgrade all currently existing prefixes will have a status of 'active'. New prefixes must have a status set.

Fixes #494.
Hm, the attribute should be just "selected", not "selected=selected", right? See for example http://www.w3.org/wiki/HTML/Elements/option.