Add thumbnail tests for two sizes image upload

[karlcow]

This is a followup on the issue #722 and its pull request #1266

1. We need to test if when uploading an image we produce the right markup in MarkDown. (js functional test)
2. We probably need to test that the images are created at the right place too. (python unittest)

[karlcow]

Was this fixed in #1557?

[karlcow]

With

webcompat.com/tests/functional/image-uploads-non-auth.js

Line 144 in dd8102a

"remove image upload button": function() {

[zoepage]

Was this fixed in #1557?

@karlcow To who was this question directed to?

[karlcow]

Maybe myself