When using *_CONFIG_FILE env vars, allow temporary edits to the configuration

Created by: slimsag

In some cases when using the EXTSVC_CONFIG_FILE, SITE_CONFIG_FILE, etc. env vars it is useful to allow temporary edits to the configuration instead of rejecting it.

In particular, this is useful when:

1. Testing a configuration change quickly before adding it to your configuration. e.g. this is important when Sourcegraph is not deployed with zero-downtime and several config edits would result in a lot of downtime.
2. A script of some sort updates the configuration in some automatic fashion.

Reported by https://app.hubspot.com/contacts/2762526/company/407948923