Page locking/collision detection

[kaedroho]

Add the ability to lock pages in the Wagtail admin to help prevent two editors editing the same page at the same time.

Or just add a message at the top saying who is currently (or has recently been) editing the page.

[davecranwell]

We had this with our previous CMS, but locks weren't "hard" you were just warned about the other editor. Part of the reason locks weren't hard is because this would have required a more robust system to unlock a page when users went afk without unlocking.