Allow user to register for a particular version

[hol430]

From @jbrider. Currently, users can only register for the point release of old apsim, or latest version of next gen. Once registered, any version can be downloaded from the downloads page. On the other hand, maybe the registrations shouldn't be associated with a particular product or version. If that were the case, we could remove the product, version, and platform inputs from the registration form. We would also then remove the download link from the registration email.

@hol353 - any thoughts? Does a registration and license agreement need to be tied to a specific product or version? This isn't really a high priority, but might be nice to revisit the DB schema for the registrations DB at some point.

[hol353]

Perhaps you are right. Registrations shouldn't be associated with a product or version, particularly as this would simplify things.

Revisiting the db schema at some point is also a good idea. Currently (I think) everything is organised by downloads with lots of duplicates if someone downloads the same product version multiple times. Ideally we should have a registration db table (one record per registration - email as unique key) that keeps track of products and versions that have been downloaded by that registered user. A user registration should be separate from downloads. A user only registers once, but can download whatever products and versions they like. I should be able to query what a user has downloaded by some simple sql.