Add ability to enter Compatibility/Test information by extension version

Right now the Compatibility field is a free form field which is hard to maintain. For example if you say "Tested with 5.0", it only applies to the current release version but if you release a new version it's no longer true.

I believe compatibilty information should depend on a version and thus the ExtensionVersionClass should have a compatibility field and we remove the global compat field.

In term of UI display, I'd propose:

• Display compat info in the release notes section at the bottom
• Display the latest version compat info where the current compat info is displayed.