Year filter for documents

If a corpus has an exact date for documents, we can use the date filter which shows a calendar widget.

If the corpus only has year data, we use a range filter which slows a slider. For a corpus like DIOPTRA-L, with a span of 15 years, this is still usable, but for a corpus like DBNL (https://github.com/UUDigitalHumanitieslab/I-analyzer-django/pull/68), with a span of 700 years, this is really awkward. Most of the data in DBNL is from the 19th century, but actually filtering on any period within the 19th century is genuinely hard.

To quote MDN:

> [``](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input) elements of type range let the user specify a numeric value which must be no less than a given value, and no more than another given value. The precise value, however, is not considered important. This is typically represented using a slider or dial control rather than a text entry box like the [number](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/number) input type.
>
> Because this kind of widget is imprecise, it should only be used if the control's exact value isn't important.

My suggestion is that we should support a year filter - essentially the one I introduce for corpus metadata in https://github.com/UUDigitalHumanitieslab/I-analyzer-django/pull/64