Dropdown options added, need basic chart javascript for now
fix so that empty names just use filled in name
Depend in viewhelpers/form instead of basiccharts/form
Fix bug when managing dashboard that has no allowed filters
basiccharts_filters_form was moved to ckanext-viewhelpers and renamed
Set default user_filter_fields and user_filter_names

They're needed all over the code. Instead of changing every place to test if it
exists before using, I'd rather set up a default value ([]).
Remove mistaken colon
Forgot to add the colon
Make jslint happy
[#3] Use filter parsing from ckanext-viewhelpers
[#3] Remove unused filter parsing method
Style the dashboard's filters
[#3] Set the filters using ckanext-viewhelpers
[#4] User is able to remove filters

To do so, I had to change the HTML structure to use <select> instead of
<input>. This also makes our code more semantic, and allow the options to be
seen even without JavaScript (which isn't very useful, as the user needs JS to
actually change the filters).
[#4] Properly indent the HTML
Set the dashboard-dropdown's width with CSS instead of JavaScript
[#5] Always allow the user to add another filter

We do so by always adding an extra filter dropdown at the end of the current
filters.
Fix bug with filters' labels not appearing when there was only a single filter

The problem is that with a single filter, Python receives ```user_filter_fields
= ''```. Then we tested ```if not user_filter_fields```, which is True when
user_filter_fields is an empty string. We'd set ```user_filter_fields = []```
in that case.

The fix is to simply check ```if user_filter_fields is None``` explictly.
[#5] Fix problem when saving dashboards with filters

As we changed the HTML element of the filters' dropdown to a <select> and not a
<div> in 7903f3, and it was inside the dashboard edit form, we ended up sending
their values together when editing the dashboard.

To fix it, I had to move it to outside our <form>. This was a bit hard, as I
had to detect when we're in the dashboard edit form to enable
dragging/resizing/removing widgets from other ways.
[#5] Disable filtering when editing dashboard