Include thread json in thread export methods

[B13rg]

Issue #1082

Creates a new way to export saved threads that includes the thread media and a json file with post data

Takes the list of ChanPosts associated with a thread and exports the post data in json format along with non-thumbnail media from the thread.

Saves everything in a zip in the format <chan>_<board>_<thread no.>_json.zip

Renamed the export types to be hopefully clearer:

[K1rakishou]

@B13rg
LGTM.

The only issue is I wouldn't recommend using ChanPost as a placeholder for converting stuff into json because that class will probably be changed soon so the json output will change too. Ideally, you need a new class, something like ChanPostDataToExport, (and also similar classes to all child ChanPost's child classes) then map them into these new classes and only after that convert them into json.

But if you are fine with a possibility that ChanPost might change in the future then I can merge this PR.

[B13rg]

I'm totally fine with it changing, since it's just an export it should be fine.
If you have plans for eventually allowing importing of posts, I could use the themeparser import/export class for reference to be merged alongside the ChanPost changes