Add option to not resolve committers to their account display name

Created by: kevachen

When viewing a repository on Sourcegraph, the commit history shows a user's display name if they have a user account in Sourcegraph. I'd like the ability to disable the functionality that resolves a committer to their user display name in my Sourcegraph instance and use the author information from git instead.

Display names are mutable, which makes it difficult in an organization to find out who made certain commits if the display name is changed to something arbitrary. I believe that Sourcegraph falls back to git author information when there's no matching user account- it would be great if we could set that as the default for all commit authors.

Requested by https://app.hubspot.com/contacts/2762526/company/693777177