feat(db): implement db.convert()

[jniles]

This commit implements db.convert() to convert hexadecimal strings on objects into binary representation for insertion into the database. It is demonstrated on both the patient groups controller and enterprises controller.

It is used as described here: #321 (comment)

let data = req.body;

// do some logic

// convert the 'uuid' and 'debtor_uuid' properties to binary using
// db.bid()
data = db.convert(data, ['uuid', 'debtor_uuid']);

// now I can db.exec('INSERT...') or db.exec('UPDATE ...') easily.

It also contains a tiny bug fix by making sure location_id is wrapped in a call to BUID(). I don't think anyone was using this API yet, but it would result in binary data being leaked to the client.

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[jniles]

@sfount, could I get a review?

[sfount]

Code LGTM.