Allow version checker to display information on multiple releases

The version checker needs to be able to display both the urgent security/bugfix release of the current branch as well as an optional update to the next feature release. This is necessary to provide an easy path from 3.0.8 to 3.1. The version checker should probably also use a subdomain of phpbb.com to avoid problems during server maintenance windows. Additional information on the release (e.g. short list of new features/changes) should be made available in the ACP as well. If the QA flag is enabled it should additionally list Beta and RC releases.