R1:4180b337cf7e

Add a "{src ...}" Remarkup rule to provide a more flexible way to reference source files in Diffusion

Summary: Depends on D20538. Ref T13291. We now recognize full source URIs, but encoding full URIs isn't super human-friendly and we can't do stuff like relative links with them. Add `{src ...}` as a way to get to this behavior that supports options and more flexible syntax.

Test Plan: {F6463607}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13291

Differential Revision: https://secure.