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.
old names still here. by the way, I think there is an easier way to initialize this by just applying the class 'datepicker' to the input (we use this all over the place and I think there is some generic initialisation somewhere) - so perhaps these initialisers could be removed altogether.