Create an 'Allow Unsupported Platforms' flag and restrict unsupported platform use otherwise

To help encourage customers to run on Supported Platforms, it would be great if by default a new JIRA instance would only run on supported databases / operating systems, JVMs, etc.

This would mean when setting up JIRA for the first time and running on an unsupported environment, it would fail fast and clearly explain why. I.e. "We currently do not Support this JVM, please try running one of the JVMs listed here <supported platforms link>". From here customers will be guided onto best practices immediately, rather than much further down the track when it's more difficult to change.

However including a flag to allow people to opt out of this will still let those admins who understand the risks and for whatever reason need to run on an unsupported platform.