Generate and edit config through CMS UI

**- Do you want to request a *feature* or report a *bug*?**

Feature

**- What is the current behavior?**

Users must manually create a config in yaml

**- What is the expected behavior?**

Users should be able to configure the CMS through the UI. The output is still just a yaml config file that is version controlled.

Considerations:
* Allow disabling through manual config
* Strong warnings for changing certain fields, e.g. anything under `backend`
* Provide access through the top right settings menu
* When settings editor loads, compare deployed config and notify user if any changes are awaiting deployment.

Start by tracing the CMS top-right settings menu and how users currently supply the YAML configuration. Define the UI, warnings, deployment-change notification, and manual-config disable path before implementation; done means configuration can be generated and edited through the UI while remaining version-controlled YAML.

Written by the indexing model from the issue text.