getEffectiveBuildDefintion should be removed from ImmutablePlan

Currently we have two methods: one that returns configuration specific to a plan and one that always returns configuration for master.
What we need is one method that returns merged configuration - patched master's configuration for branch and plain master's configuration for master.
At the same time, we want to be able to edit the branch specific configuration and save it without accidentally snapshotting master's configuration.

This is needed for BranchIntegration configuration - it resides in Branch-specific configuration and is not available in the build context.