Restore editor behavior to Diffusion and support "\" shortcut

Summary:
Ref T13515. This restores the "Open in Editor" behavior to Diffusion, and makes "\" work there.

The URI pattern is now sent as a structured template to the client, so the code will work properly if a file path contains "%l".

Test Plan:

• Clicked "Open in Editor" and pressed "\" in Diffusion when viewing a file.
• Clicked a line, hit "\", got the file opened to that line.

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21149