Update documentation to reflect management console

Created by: slimsag

• Update all docs to reference the management console.
  • https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/configure.md#update-site-configuration That section should now link to documentation on docs.sourcegraph.com about the site admin config page + the management console; That section should probably be updated with information about how to access the management console (port-forwarding, changing the password in a k8s context, etc.)
  • All references to the site configuation file in the rest of https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/configure.md need to be updated to stop refering to the config-map
  • The migration path from the old https://github.com/sourcegraph/deploy-sourcegraph/blob/181c382d7d737cc0178bb94c6eb98da5b492a0de/base/config-file.ConfigMap.yaml to the management console needs to be documented
  • https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/configure.md#add-license-key
  • Make any necessary updates to deploy-sourcegraph and documentation there (e.g. migration steps if any).
• Clearly document that some properties (e.g. externalURL) require the frontend server to be restarted. This is a known bug: https://github.com/sourcegraph/sourcegraph/issues/517
• Clearly document how migration occurs:
  • Add management console section to https://github.com/sourcegraph/deploy-sourcegraph/blob/master/docs/sourcegraph-3.0.migrate.md
  • Users should expect to now visit the management console for certain things
• Update CHANGELOG to reflect management console change.
• Clearly document how the management console works
  • The password is auto-generated, accessible via site admin area.
  • The SSL cert is self-signed, must accept in browser.
• Is a blog post desirable? cc @sqs
• Document how to access dogfood management console. https://sourcegraph.slack.com/archives/C0EPTDE9L/p1547161784030100

Note to self: See also notes about documentation under https://github.com/sourcegraph/sourcegraph/pull/966