enable/disable features in config.json
As per https://docs.google.com/document/d/1Kn-mz2dDce9Cqc4oUTl5yJXGvmTlky1_KezuwUg58x0/edit#
Replaces:
* enableLabs setting
* 'override' flag in labs
* 'default' flag in labs
Un-feature-flags matrix apps since this was now overidden to be
enabled.
