Our extra_data support in the API has previously been limited to

storing primitive values (strings, booleans, integers) associated with

top-level keys. It didn't allow for any complex JSON data.
This change introduces two new methods for storing and managing more

complex JSON data. Clients can now set the extra_data:json field in

POST or PUT to a serialized JSON Merge Patch, which is a JSON document

that will be merged into the root of extra_data. Keys mapped to

dictionaries in the document will merge the dictionary from the JSON

Merge Patch. Arrays will replace arrays. Any key set to None will

remove that key from the dictionary. This is a pretty good,

straight-forward way of working with JSON data.
For more advanced usage, clients can set extra_data:json-patch to a

JSON Patch document, which is a list of operations to perform on

extra_data. This allows for setting entire JSON structures on

dictionary keys, at the end of an array, or even at specific array

indices. Operations can remove parts of the JSON document, copy/move

data. They can also sanity-check the presence of existing data, aborting

the entire patch if not present.
Both forms of patches prevent replacing the entire root resource itself

(though every key can be specifically deleted), so that clients don't

accidentally remove all state. They also obey any access restrictions

imposed by extensions, preventing reading from private parts of a tree

and writing to private/read-only parts of a tree. Keys with double

underscores (__keyname) are also still supported, and can occur at any

level of the tree.
Documentation on all of this will be added in a future change.

Unit tests pass.
Manually tested that both forms of patching work.