Suggestion: expire ownership of devices

[moshe-braner]

The problem keeps coming up with registration of second-hand devices when the previous owner has not un-registered. To avoid the need for "staff" intervention, I suggest "expiring" devices after some time (e.g., 2 years) from registration or modification of the registration record. After that time, a new owner would be allowed to register the same device. Unless and until that happens, the old registration would keep "working" in all other ways.

The essential development bits to implement this are:

• add a date field to the DDB schema "devices" table.
• update that date field to "today" when a device record is created or updated (or "renewed").
• when checking whether the device is already registered to somebody else, check that date, if more than 2 years ago then do allow registering by a new owner.
• when actually registering a new device record for the new owner, delete the existing device record with same device ID.
  The rest is optional. E.g., a "this device registration has expired" notice plus a "renew registration" button would be nice, but can start with just some generic text on the page explaining that one must re-save the record every year or two to avoid it expiring.

[skizzofly]

check out pull 57
#57
was not so difficult.