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:

**Edit**: runs are ok for [amo](https://ci-addons.allizom.org/job/amo-build-branch/43/) and [mkt](https://ci-addons.allizom.org/job/marketplace-build-branch/20/) on jenkins

**Edit2**: the ```|f``` helper behavior has changed in the latest version of Jingo. This change is discussed in great details: https://github.com/jbalogh/jingo/pull/54#issuecomment-34615042

TL;DR:
- ```{{ _('<strong>This won't be escaped, is seen as safe, and doesn't need a |safe at the end</strong>') }}```
- ```{{ _('<strong>this will be escaped</strong> {0}')|f('even if the <b>end result</b> is marked as safe')|safe }}``` (the solution is to move the ```|safe``` inside the last paren to mark ```|f```'s input as safe)

Key takeouts:
- using Tower, a ```{{ _('a localized string') }}``` is a Markup object, thus safe, and doesn't need a ```|safe``` at the end to not be escaped
- a Markup object (returned either from ```{{ _('a localized string') }}``` or ```{{ 'a string marked safe'|safe }}``` is safe and will never be escaped
- a Markup object string interpolated with anything else will always escape the input (unless the input is marked as safe/is a Markup object), even if you tack on a ```|safe``` on the end result (safing a Markup object is useless, a Markup object IS safe, and it won't unescape its content)
- previously, using ```|f``` unsafed the string, then interpolated it with the inputs (which weren't escaped at that point), and the final result could then be marked as safe so as not to escape it. This is not the case anymore