Refactoring basiccharts_filters_form.js
Remove useless code in basiccharts_filters_form.js
Remove mentions to "filter_values" in basiccharts_filters_form

This makes the code more reusable, as you can put in any template you'd like,
and we don't make assumptions on its inputField's name.

We're still assuming that there's one (and only one) input[type="text"][name]
in the template, which is bad, but it's better than it was before.

This change was needed because in ckanext-dashboard we use this code, but with
another "name".
[#23] Move everything Python-related to ckanext-viewhelpers
[#23] Move basiccharts_filters_form
[#23] Extract filters parsing