Event range request improvements

[carrythebanner]

Updated how event ranges are handled:

• Moved max day range value to config
• Made day range requests inclusive of start & end dates
• Front end now fetches the expected number of events with the "show more" button, if both startdate & enddate were provided by the user; fixes "Show additional events" button always fetches 10 more days, regardless of the previously requested range #760
• Moved front end default days range to config (helpers.js)

So now:

• Events endpoint pagination.range returns 1 as expected when startdate and enddate are the same date; fixes Events endpoint daysInRange() is off by 1 #567
• Front end now defaults to 10 day range request as originally intended (previously fetched start date + 10, i.e. 11 days)

Also:

• Added prev to Events endpoint pagination object, alongside next

[ionous]

re: range in general. i think i can see it either way: an additional number of days to see. or, the absolute number of days to fetch.
it might be worth adding a comment to the const EVENTS_RANGE to indicate that its the latter; and so therefore always needs to be 1 or greater. ( the server side could potentially get the same sort of comment on MaxRange, too; but maybe less important there because its able to validate > 0 when the request is submitted. )