Add daily reminders for meetups happening today

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This PR adds the capability to post daily reminders for meetup events happening today to slack:

Daily reminders

Fixes #1

Extra

It also adds the capability to replace the houly 60 minute event reminders in the Tampa Devs slack that are currently going to #meetup-reminders to address #4 (though actually replacing those are outside of the scope of this PR), and makes some small tweaks to the weekly event reminders (like posting events happening just this week instead of two weeks out).

The main.rb script would now be invoked as ruby main.rb [--daily|--hourly|--weekly]. By default ruby main.rb with no arguments defaults to main.rb --weekly as to maintain backwards compatibility with how the script is currently being invoked.

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cc @chtzvt I am curious for your feedback on this 👀

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Here are what the hourly reminders would look like:

And weekly reminders:

There are some minor styling changes to weekly announcements (changing the header text to a header type). I also changed it to show events just this week, as opposed to the next two weeks, and changed the header text to reflect that change.

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@chtzvt I'd really like to make this feature happen 🙂

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@iBotPeaches might you take a look?

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After chatting a bit with Charlton yesterday, I'm going to add a --destinations argument to specify which slack groups we want the daily reminders to go (basically just to TD or to TD+TBT+TBUX). Also, I'll add a new GitHub Actions workflow to run on a daily cadence.

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Couldn't figure out how to rebase the commit cleanly, so I'm moving this PR over to #7