Image associations are not returned correctly

[tsauerwein]

Reported in https://github.com/c2corg/v6_ui/issues/861#issuecomment-262714371

In https://api.demov6.camptocamp.org/images/188564, it misses the association to the image 208579, as they are associated each other, and there is no hierarchy in the image association.
But as the association to 188564 appears in 208579, it seems the association has been migrated correctly in the BD, and it would just be a problem in the API.

[cbeauchesne]

Note for later : association A/B is not the same as B/A.

For images, both are possible, and are (data POV) equivalent. But only A/B is missing on B page.

[cbeauchesne]

https://forum.camptocamp.org/t/nouvelle-interface-bugs-questions-suggestions/241148/161?u=charlesb