Provide panels for task rendering

Previously, we used the same html code for pdf generation and for
integration in the view_only output. The view only output contained a
full html page (with head and body tags).

Now we separate the html page and the body's content that is now in a
panel.
The html output now also handles multiple task rendering.
Use the panels for task rendering

We use the modifications carried by the previous commit.
Update a docstring
Ensure the current year is available in year widget

If no invoice has been created, we've got no year entry in the year
select widget.
Add a form schema for bulk pdf export

In order to be able to print a large number of invoices, we provide a
bulk export.
Here we provide the colander form schema
Provide a bulk pdf export view

A form is provided in the manager's invoice list.
It allows to ask for a range of invoice numbers.
A view is responsible for showing form errors, query the database and
return a pdf export of the queried invoices (and cancelinvoices).
Add the export button in the manager invoice list view