Blank upgrade page afer upgrading docker manager

[riking (Kane York)]

@featheredtoast, is this your stuff?

Introducing .discourse-compatibility: pinned plugin/theme versions for older Discourse versions

Hey everyone , I just merged a new feature that will help plugins and themes pin certain versions when installed on older Discourse instances. You can now include a .discourse-compatibility file at the root of a plugin or theme repository, that designates what to checkout when installing on older Discourse versions. Rationale It sucks to remember which plugins and themes are compatible with which Discourse versions. As an admin, it should be possible to easily scan those changes and fi…