Support go-import meta tags

Summary:
GoGetMetaRepositoryExtension is a class which adds custom handling
of diffusion requests that include ?go-get=1 in the url.

To make this work, a custom extension mechanism was added to
DiffusionRepositoryController in D775

Test Plan:
see test plan in D775

Depends-on: D775

Reviewers: dduvall

Differential Revision: https://phabricator.wikimedia.org/D776