Allow URL parameters to be passed in to `previous` and `next` links

[curtisgibby]

So that filtering variables can be passed from the current month view (e.g. January 2021 having a URI string like events/calendar?filter_key=123) to the previouslink (events/calendar/2020/12?filter_key=123) and the next link (events/calendar/2021/02?filter_key=123)

[dereuromark]

Can you make a PR of what you mean?

[curtisgibby]

Fixed in #17;