The `plugin:pull_compatible_all` task is intended to take incompatible plugins and downgrade them to an earlier version. Problem is, when running the rake task in development/production environments, the plugins have already been activated. If an incompatible plugin raises an error in `plugin.rb` then the rake task will be unable to start.
This commit centralises our LOAD_PLUGINS detection, adds support for LOAD_PLUGINS=0 in dev/prod, and adds a warning to `plugin:pull_compatible_all` if it's run with plugins enabled.