This introduces djblets.util.json_utils.json_patch, an implementation

of JSON Patch (RFC 6902). This is another, more advanced method of

patching JSON-serializable data, sort of like with JSON Merge Patch but

with the ability to order operations, copy or move data, sanity-check

values before applying a patch, set values to null, and manipulate

specific indices of arrays. It's a good substitute for JSON Merge

Patches when more specific modifications and sanity-checking is needed.
JSON Patches consist of a list of operations to perform. These are

add, remove, replace, copy, move, and test. All operations

must pass in order for a patch to apply.
This implementation goes beyond most implementations by providing read

and write access control for keys. This allows callers to prevent

reading certain keys or writing to others, limiting the damage that, for

instance, an API call can do.
More information on JSON Patch can be found at http://jsonpatch.com/ or

https://tools.ietf.org/html/rfc6902.

Unit tests pass.