need central defaults for flags

At the moment the default behaviour for flag-controlled features (ie when there are no relevant rules) is set directly in the code: if they get None back, they will either do the behaviour or not.

This has a couple of problems:

 * It's not possible to automatically accurately say on <https://launchpad.net/+feature-info> what the default is.
 * As a feature matures, it may be desirable to garden-out the rule and make it default to being on, but still with the option to turn it off if it misbehaves. With no central default this would require updating all the code which would be unnecessary churn.
 * Naming/semantics of feature flags is inconsistent because some turn things on and some turn things off.