[WIP] New post settings menu

References #3932

This is purely a visual implementation of how the new PSM will open & close. Aside from the open & close functionally, it is broken. You cannot edit any post settings here.

It is only intended to show how it should work. There is a crucial details to be considered here, so I have [commented](https://github.com/TryGhost/Ghost/issues/3932#issuecomment-54166404) on #3932 with that.
Yep, second tab. I did start working on a transition for the second page, but I'm really not sure how it should be done, so didn't want to get too ahead of myself.