Disable all Edit Flow modules by default for new WordCamp sites

[coreymckrill]

In #1226 the eventual outcome was that the Edit Flow plugin was installed on the WordCamp.org network. The intention was to network-activate it so it would be available to everyone. However, the current state of it is that it is not enabled on individual sites by default. Instead, an organizer must request that it be activated for their site.

The missing piece before we network activate Edit Flow is described thusly:

...we'll need a snippet that we can add to an mu-plugin that will disable all EF modules by default, so that it doesn't have a negative performance impact on sites that don't want to use it. Camps that want to use it can then manually enable whichever modules they'd like.

An alternative to an mu-plugin might be to modify the new-site routine in the wcpt plugin so that the EF module states are set up at the time when a new site is created.

[iandunn]

WP 4.9 will give us some new options for plugin capabilities, so I think #3202 will be a better and more general purpose solution to this.