server: use /etc/sourcegraph/gitconfig

Created by: keegancsmith

This follows the same pattern we have for SSH and netrc. It makes it much more convenient for administrators to specify a git config for the Sourcegraph Docker image.

Note: Cluster based deploys don't support this method, but those have ways to put the configuration file in the correct place via resource definitions.

Part of https://github.com/sourcegraph/sourcegraph/issues/658