Fix ambiguous relation selector
When there are ambiguous relations clicking
to select one or the other does nothing.
This bug affected only our production server,
devel and debug worked well. This is weird,
and I think we are encountering the strange
behavior described in
http://yuilibrary.com/projects/yui3/ticket/2532993
or similar: in some situation (e.g. our production
server, where js files are minified/compressed)
using the "on" method on NodeList does not work.
I am confused about the real reason for this bug.
However, the documentation says that we should
avoid using NodeList.on, in favor of using event
delegation, that is exactly what I did in this
branch. Tests are not included: we have a specific
card for adding tests for the relation module.
R=benji, gary.poster
CC=
https://codereview.appspot.com/7177050