Re-download images

**wallabag-app master branch**

I have the option "Cache images locally" checked in the app (not in wallabag web). Sometimes there is a problem downloading the images and I can't see it in some articles. Related https://github.com/wallabag/android-app/issues/444#issuecomment-283505478
If I delete the article and add it again usually works, so there is no problem with the article.

I propose to implement 3 solutions:
1) When you update the article list try to re-download missing images (I mean partial, I don't know if full update tries to redownload all images but I think this is to much overhead)
2) If you enter in one article and images are not stored locally, try to show the online version, and then save them locally.
3) Add a button to refetch the article (download both, text and images).