Raw repo contents API
Created by: slimsag
This PR adds the API for fetching raw file contents. This is needed for language extensions to query repository contents via HTTP.
For a high-level overview of what this is capable of, see the extensive documentation / examples inline: https://github.com/sourcegraph/sourcegraph/pull/800/files#diff-1e4bf7dc9a47bb71ad96285d815d6d85R19
A secondary goal of this change is to serve raw files for a "raw code view" similar to what GitHub has. However, this PR does not implement such a UI feature -- just the endpoint that would handle it.
Related: #874
Closes #606 (closed)
This PR updates the CHANGELOG.md file to describe any user-facing changes (there are none).