[#82675948] ember catch-all route will only respond_html

This makes it easier to find bugs for URLs that do not exist.
At least for formats other HTML they will cause Rails to serve
500 errors.
[#82675948] add missing chosen and select2 images

This stops the 404 when a user tries to add contributors
on staging or production (or uses a select2 component).
Merge pull request #602 from Tahi-project/ember-route-404

Ember route 404