Display per-month standings for May, June, and July to enable monthly competitions

[adamengst]

As a runner, I'd like to be able to focus on a particular month, rather than on the overall competition, since I may be competitive for a while, but not for the entire period. Plus, if I come back from an injury, I may not have enough time to compete overall, but could for a final month.

Initially, I was thinking about this as a separate page for each of May, June, and July—the three full months of the 2022 Challenge that will run from April 16 through August 14. However, what might work even better is would be month buttons at the top of the main page that would filter all the stats below so they included only the month in question. I haven't yet decided if there will be prizes or just bragging rights associated with the monthly victories. If the latter, then April and August could be included even though they're not full months.

Acceptance criteria:

• Runners should be able to see the home page standings for any month at any time.
• Standings should reflect only runs recorded within the inclusive dates of the selected month.
• Ideally, race pages should also offer the date filter buttons so people can see if they're close to being in the top three.

[SteveDesmond-ca]

Implemented as of 33a805b -- a new button bar will appear at the top of the leaderboard, overall, and course results pages once results start rolling in at the beginning of May