Tie all the pieces for symbol cross-references together

Summary:
This makes symbol cross-references work in Differential. You need to do a little
legwork but I'll document that once the change has baked for a little while.

Basically:

• Projects are annotated with indexed languages, and "shared library" projects

(for example, symbols in Phabricator should be searched for in Arcanist and
libphutil).

• When we render a changeset, we check if its language is an indexed one. If

it is, we invoke the decorator Javascript.

• The Javascript takes you to a lookup page, which either gives you a list of

matching symbols (if several match) or redirects you instantly to the
definition.

Test Plan: Clicked class and function symbols in a diff, got jumped into
sensible sorts of places in Diffusion.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 980