site_config: update editor content after loading from backend

Created by: unknwon

Upon every click of "Save changes" we are getting back the up-to-date contents for site configuration (which may or may not be the same as the current editor's content due to formatting). It creates a confusing UX that our web app keeps complaining about the inconsistent state (and offers to save again and again).

This PR updates the editor's content whenever we fetched it.

Fix for https://github.com/sourcegraph/customer/issues/979

Test plan

Manual e2e test since this is a UX issue (the license key won't work alone without its private part to be used as the validation key).

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.