* Rename `app` helper to `resolve`, deprecate old version
* Rename $this->app to $this->container in service providers
We no longer couple Flarum\Foundation\Application to the Laravel container; instead, we use the container separately. Changing our naming to reflect that will make things clearer.
Policy application has also been refactored, so that policies return one of `allow`, `deny`, `forceAllow`, `forceDeny`. The result of a set of policies is no longer the first non-null result, but rather the highest priority result (forceDeny > forceAllow > deny > allow, so if a single forceDeny is present, that beats out all other returned results). This removes order in which extensions boot as a factor.
