E-library documents can be filtered by date range

Introduces start & end date inputs for filter.

Somewhat unrelated: wrapped the filter inputs in `.form-group` `<div>`s to make styling a bit easier and fit in with bootstrap.
the parameter sent from the form is called document-created-start (analogically for the end date), so the filter ends up not being applied. I'd suggest sticking to the document-date-start convention, because created_at is the name of another field and it will get confusing in a bit by which one we're filtering.