Tighten access to deploy-sourcegraph-dot-com

Created by: ryanslade

Currently, most of us can merge changes and even push directly to the release branch on https://github.com/sourcegraph/deploy-sourcegraph-dot-com

As we move to allow private code on Sourcegraph.com we should be tightening up access to this repo so that it's harder to make changes that may expose private code.

I propose that, at a minimum we:

1. Disallow pushes directly to release
2. Enforce at least one review from another member of the team before merge