frontend: merge SITE_CONFIG_FILE
Created by: keegancsmith
This commit adds support to expand SITE_CONFIG_FILE into multiple files. The files are merged by writing out the key values from each file into a new JSON Object. We include comments so an admin can tell where the keys come from.
The purpose of this commit is so we can seperate out the secret keys in the site config from the more general keys. I hope to make the DX around updating most of the site config much easier. IE most of the config lives in deploy-sourcegraph-dot-com repo.
Note: there is no validation done at read time. This is left for site config parsing time.
Part of #21220 (closed)