Add a comparison controller

This controller implements a "compare" URL which will report the files
which have changed between two commits.

For example, http://localhost:8080/api/compare/v1/c3608e5/master returns

{"renamed": [], "removed": [], "changed": ["study/2680/2680.json",
"study/2681/2681.json", "study/2690/2690.json", "study/284/284.json",
"study/605/605.json", "study/704/704.json"], "added":
["study/2697/2697.json", "study/2698/2698.json", "study/2699/2699.json",
"study/2700/2700.json"]}

This is intended to be used by whatever implements a full-text index
of our NexSON files. This will allow that system to ask "what has changed
since X" and then update the full-text index by processing those files.
Actually add the search controller which was forgotten in f944e76