When users click a symbol in Differential to jump to the definition, include path/line context

Summary:
Ref T13047. In some reasonable cases, knowing the path and line number where a symbol appears is useful in ranking or filtering the set of matching symbols.

Giving symbol sources more information can't hurt, and it's generally free for us to include this context since we just need to grab it out of the document and pass it along.

We can't always get this data (for example, if a user types s idx into global search, we have no clue) but this is similar to other types of context which are only available sometimes (like which repository a symbol appears in).

Test Plan: Command-clicked some symbols in 1-up (unified) and 2-up (side-by-side) diff views with symbol indexes configured. Got accurate path and line information in the URI I was redirected to.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13047

Differential Revision: https://secure.phabricator.com/D18936