Create layout visibility conditions and "create links" block

[argiepiano]

D7 OG contained a few Panels selection rules, and one relationship plugin. These definitions have been moved to a file called og_og.inc, which doesn't do anything in Backdrop.

We should create parallel OG Layout visibility conditions and relationships so that they have the same functionality as the ones available for Panels in D7.

PR coming.

[argiepiano]

PR #119.

This PR is limited to only the Layout visibility plugins. While I tried porting the layout relationship OG membership from a group node and user, this is being blocked by a bug in Backdrop's core. See backdrop/backdrop-issues#5482

This PR adds 4 visibility rules to layout. See attached. I've tested very thoroughly and it's working. I'll wait to merge to see if anyone else wants to test.

[argiepiano]

I just added also a new block, "Content create links", that existed as a Panels content type in D7. Now it's possible to add this block to a normal layout. The block creates a direct link to the add form to add content to the group node being viewed.

Important: in order for this to work properly Entity Reference Prepopulate must be enabled. Otherwise the group will not be pre-selected in the group reference field in the form. See this video tutorial.

This fulfills the enhancement request #28.