This introduces djblets.util.json_utils.json_merge_patch, an

implementation of JSON Merge Patch (RFC 7396), which is used to apply a

set of changes to a JSON-compatible data structure. Merge Patches allow

for adding new values (which may be complex JSON objects) to

dictionaries or arrays, remove keys from dictionaries or arrays, or

replace existing values.
This has additional support for access control around keys, preventing

certain keys from being replaced or added, optionally aborting the

patching process if encountered.

Unit tests pass.
Tested manually with several targets and dictionaries.