Allow authors to cross-post to all Make blogs

[iandunn]

If a person has a Contributor (or higher) role on any Make site, I think they should have the ability to cross-post to any other Make site.

Otherwise people have to be manually added to each individual site, which is tedious and adds an unnecessary barrier to communicating.

(For contributors, their post would still need to be approved by an Editor before the cross posting would happen, of course)

One way to do this would be to programatically add all users to all sites, but there's probably a more elegant way to do it with map_meta_cap or has_cap. It would need to make sure that all of the sites appear in the list that o2 shows when you type +.

Sites that are marked as private, archived, and deactivated should be excluded from this functionality. Users should not be given access to browse wp-admin of the other sites, etc. It should only allow them to cross-post.

[dd32]

In 9352:

o2 Posting Access: Add an additional plugin to customise o2 crossposting.

This plugin allows crossposting to all same-status network sites running o2, whether you have posting access to the destination site or not.

See #4501.

[dd32]

The above plugin appears to handle this, I'd appreciate any review of the plugin before I activate it on make.w.org/*

(I'm also delaying activating it until I've let it sit overnight and I give myself some time to have that eureka "I shouldn't have done that.." moment)