Set a site-wide default layout.

[robinlinden]

To create a page without a layout you can use layout: null.

---

This change is 

[iphydf]

---

Reviewed 18 of 18 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

---

Comments from Reviewable

[cebe]

---

Reviewed 18 of 18 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

---

Comments from Reviewable