This commit implements most of what is required by #23, with the exception of labels processing as stated in https://gitlab.com/wgp/dougal/software/-/issues/23/designs/20200908_101516.jpg#note_408968681
It can show: - all events (this could get slow); - a single sequence; - a set of sequences; - a single date; - a range between two dates. It does not (yet) do pagination and filtering is local only.