HTH: IDOC: Allow option for restricting user sign-up

We now have two scenarios where restricting user sign-ups have been requested:
* For HTH, in order to spawn a single-user KA Lite install, it is safest to disable user creation (to tightly control what data we allow to enter/exit the install)
* For IDOC, in order to allow them to do a controlled study, they need all student accounts to be well known and follow their formatting procedures.  Adding new accounts is undesired.

To handle this, I've:
* Extended the "config package" idea such that a single install can contain multiple "config packages"
* Added a "config package" for restricting user activity.  This currently toggles things so that a new cache is made, and the setting is used to hide / disable sign-up.  It is implemented as a config package for future iterations where, for example, we may want to toggle other settings and disable other functionality based on this.

I'm somewhat annoyed by this design, but also wanted to avoid implementing a per-feature setting for toggling on/off.