[#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
Rename Display Name -> Label, and allow translating it
Allow translating "More" term
Only show in the Allowed Filter's dropdown filters that can be used

At the moment, we're only able to filter by textual columns. Hopefully soon
we'll be able to filter by dates as well.
Refactor getting field list, removing duplication on the code
[#6] Allow the user to select timestamp fields to filter by
[#5] Remove duplicated allowed filters

As we're allowing the user to create as many filters as she wants now, there's
no reason to create a dashboard with multiple allowed filters to begin with. If
the admin creates one, we remove every repeated allowed filter, leaving just
the first occurrence.

Ideally, this would also work in JavaScript. I mean, if I have a filter for
"date" already created, when I add another filter, "date" doesn't appear on my
list of possible filters. Unfortunately, this would require a bit more work,
so, for now, this solutions is good enough.