Add a "Missing repository, expected "{src repository:path ...}" or "{src path repository=...}" in: {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:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13291

Differential Revision: https://secure.phabricator.com/D20539