Integrate advice plans

Pulling advice_plans and decision_trees into frontend
This is used by advice_plans and decision_trees (our understanding was) to manage the different engines that make use of decision trees.

Here we removed the functionality as it is using unnecessary Tool models (from public_website) at the minute as we only have advice_plans in use currently in frontend. Rather than rip out the methods and method calls we left this infrastructure in place as we expect it to be used as soon as one of the other two engines are pulled in.