JSON Import/export

[xet7]

Moved to here from phildini#29

From @phildini

As a user, I want to be able to export a JSON file of my complete contact book, and import it again later.

Should also help for moving between hosted/sandstorm

From @paulproteus

+1

What I would do for an initial implementation is not make this part of
the synchronous Django request processing chain. Instead, make it work like
search indexes via cron, and email the user when their backup is ready.

Then you can "just" use Django's built-in JSON serializers.

From @phildini

@paulproteus do you have a solution for cron working in Sandstorm? :)

From @paulproteus

No I don't! But you could instead do it in a background task, which you
could do (without celery) via
https://hendrix.readthedocs.org/en/latest/crosstown_traffic/ if you like.

From @phildini

Some more thoughts here: need to support format versioning, and should probably use the same serializers the api uses.

So export has been added by 8a99434. Probably knock out import this week, and then will feel really good about putting a version on Sandstorm.