[#5452] allow lists in resource extras

[wardi]

Fixes #5452

Proposed fixes:

remove two lines of code from resource_dict_save. This allows ckanext-scheming and other extensions to have repeating subfields and other list values in their resource schemas without hacks like converting to/from JSON (resource extras are already stored in the DB as JSON)

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

[wardi]

One more small change was added here. In current versions of ckan dicts passed as resource extras were being stored as their python repr, e.g. "{u'mykey': u'myvalue'}" even though the lower level JsonDict field was capable of storing actual JSON. I've added an extras_valid_json validator instead of the validator that was turning objects into their python repr.

[wardi]

resource extra value (JSON)	returned value before	returned value after
"text"	"text" ✔️	"text" ✔️
42	"42" ❌	42 ✔️
{"test": "value"}	"{u'test': u'value'}" ❌	{"test": "value"} ✔️
true	"True" ❌	true ✔️
null	"None" ❌	null ✔️
[1, 2, 3]	(silently removed) ❌	[1, 2, 3] ✔️

I think this can be considered a bug fix, the current behaviour is very strange for a JSON API

[amercader]

@wardi I had to tweak this a bit to adapt it to the old way validators where handled in the schema in 2.7, do you mind having an extra quick look at these to make sure nothing seems off? thanks

85656ba...8fc5ceb

[wardi]

LGTM