Refactor repository browsing into Vue: Add support for commit signature in Commit Widget as well as GraphQL API

Summary

Before we enable the Vue file listing feature flag we need to add support for commit signatures on the GraphQL API.

This will require some backend work to add it into the API and also some frontend to update the Vue components.

Details

We need to make sure to port every scenario covered by these HAML files:

/cc @andr3