Add a default 'posts' view to core

Follow-up to https://github.com/backdrop/backdrop-issues/issues/940

I decided to create this issue because I was having some trouble deciding how to properly architect a posts view for Backdrop core. The easiest thing to do would be to use a page display. But, in Backdrop would people prefer to have a layout with a block display instead? In either case, do we put a link to this page into the menu by default?

There are some questions that need to be answered around the best way to do this. Opening for discussion.