add option for allowing temp edits when using *_CONFIG_FILE env vars

Created by: slimsag

This adds two new environment variables: EXTSVC_CONFIG_ALLOW_EDITS and SITE_CONFIG_ALLOW_EDITS which when true allow edits to be made to the external service config and site config even when loading configuration via a file using EXTSVC_CONFIG_FILE or SITE_CONFIG_FILE.

For why someone would want to do this, see #4912 (closed) -- in particular this helps a large customer try out options easily before committing them (which involves some downtime as Sourcegraph must restart.) It is also something we do in dev environments today.

The reason I am working on this now is to help a customer debug an issue by setting https://github.com/sourcegraph/sourcegraph/pull/5202 on their instance.

Fixes #4912 (closed)

Test plan: Manual testing (can't be automatically tested easily) + thoroughly being tested by all devs since it is on in our dev envs. No change for users who don't set the new option, so low risk.