Real-time collaboration: Add WP_ALLOW_COLLABORATION constant.

This provides an easy way at config level to disable real-time collaboration.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11311.

See #64904.
Props alecgeatches, ingeniumed, zieladam, peterwilsoncc, tyxla.


git-svn-id: https://develop.svn.wordpress.org/trunk@62075 602fd350-edb4-49c9-b593-d223f7449a82