Registration support

[wyozi]

There should be an HTTP endpoint to allow registering a license. The endpoint should be able to receive a password (as a specifiable query param), which must match the admin-set password for that product. If the password is correct, a license is registered for given product/user and a custom ID (UUID perhaps?) is returned in the HTTP response, which should be later used by pings to identify the specific registered license.

Registering should be opt-in, but if registration is enabled it should be the only way to be able to ping the said product.