Allow developers to specify a suggested package name

[Anthony Lenton]

When submitting an app, a developer might prefer its packaged with a specific package_name, and MyApps doesn't currently allow the developer to enter this.

Also, pkgme-service needs some kind of a package name to work with, and automatic packaging happens before a reviewer has come along to specify a package name of any kind, so currently we'll need to resort to guessing, and munging the app's name into something package-like.

Ideally, the developer could enter a (suggested) package name together with other suggested fields like the version number, during the app submission workflow.