Update Jingo to 0.7 and Jinja2 to 2.7.2 (bug 963513)

*/!\* When merging, remember to update those two reqs to pyrepo */!\*

I've left the commits unsquashed for now, for ease of reviewing (well, sorta).

They should be rather tidy and do what they say in the commit message.

Tests are passing on amo locally, and will run a "build-branch" build on
jenkins for both amo and mkt.

Just a reminder: updating Jingo makes it play nice with DjDT: you can now see
the templates used (and their source) in the debug toolbar! :+1: