List past events by month

[bridgetkromhout]

This is an MVP attempt at fixing #291. I think it's enough of an improvement that I'd like to put it in place, though improvements are welcome.

What it gets right:

• showing the past events with months/dates makes it much easier to see what happened when (like say, if you wanted to see what month a past event was in, or what events were in a given month)

• easier to scan than the giant columns

• No longer duplicating future events at the end of the current year

• Getting rid of the different way to parse the past events simplifies things.

What is still not ideal:

• I would prefer it broken up by years, but that requires saving/comparing state in a way we're not doing right now.

• This isn't the DRYest way to code this (but I wanted to repeat myself because I intend to change the "past" more).

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This change is 

[bridgetkromhout]

After noting that https://deploy-preview-655--devopsdays-theme.netlify.com/events also puts dates in reverse order inside each month, I definitely think I need a different ordering mechanism than this "reverse" one.

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[bridgetkromhout]

I don't like this enough to release it as-is right now.

[mattstratton]

@bridgetkromhout Do you want to keep this PR open while you keep playing with it? Totally fine if that's the case.

[bridgetkromhout]

Yeah, for now. Thanks.