Merge branch 'release/0.26.2.4' into develop
Switch to celery 3.1.9, sparks 3.10 (for pip cache), and keep only one celery dependancy to be sure to always have latest ones (via Django-celery).
Switch to sparks 3.10.1.
Switch to sparks 3.10.2.
Bump devserver to 0.7.
`article.find_image()` is not called anymore, it will be used in views until it is validated for worker usage (needs love & test).
Fix IMG size in reading lists.
Development settings update.
Enhance the test for klass/not-klass routing in common JS code.
JS: setup async images.
create a new `article_image` view, on the road to integrate article images in the reading list.

This is a always dynamic version, which will waste some resources on the server-side. The full static (pre-computed) version needs a lot of work server-side to be sure we pick the right image, so it will wait. This version is a step in the right direction.
Use the article image in reading list :-)
version bump for 0.26.3.
Merge branch 'release/0.26.3'