Move splitSearchModes feature flag from config into settings

Created by: attfarhan

Adds an experimentalUIFeatures field into the settings schema, and moves splitSearchModes from config into settings. This allows us to turn on feature flags for individuals and orgs on instances rather than for the entire instance, which is useful specifically for Sourcegraph.com so we can get our team to dogfood experimental features.

Note: I named the field experimentalUIFeatures since we can't have repeating names in settings and config, and I figure that everything in the settings config will be a UI feature. Open to suggestions for better names.