Do not use any template in _filtering_value helper
Improve filtering form interface and functions

Allow to filter by attributes on nested associations and by nested associations themselves (selecting them from a list of checkboxes).

This seems to work, but is not very well tested.
Change filtering on "participants_to_approve" page

Remove the redundant "approved" filtering parameter on "participants_to_approve" page.
Replace "representation theory" with a better term
Experiment with order of columns in filter form
Fix error accessing org/participants/edit/...

Do not render filter form if `@filtering_parameters` variable is not set. This looks like a temporary fix, it would probably be better to always have the filter form.
Move flash messages to a partial
Add a comment
Remove useless private methods in application.rb
Use whole Participant object instead of just id

When rendering a list of participant with an edit form, pass a whole Participant object in an instance variable instead of just id.  This will allow to render form errors if save fails.
When updating, use 'save' instead of 'save!' and

render or redirect properly.
Do not validate uniqueness of full name