Await on getEntitlements in getAvailableIntervention

[mhkawano]

We have a race condition between start and getAvailableInterventions where start will not necessarily call getEntitlements before it's needed by getAvailableInterventions. This change removes the need to call start by having getAvailableInterventions make its own call.

This will not duplicate the network call, as the entitlements manager will keep one promise until reset has been called. Additionally start will not even make a getEntitlements call on unlocked content, so we need to remove our dependency on it either way.

Tested manually on the enterprise-funnel.html and enterprise-funnel-manual.html pages.