Implement /meeting-reminder command

[QuantumManiac]

Description

This PR implements the /meeting-reminder command, formerly known as /meeting reminder. This allows users to trigger a reminder for the soonest upcoming event in the channel that it was executed in. In addition, executing /meeting-reminder ping not only triggers a reminder, but also pings @channel.

This PR closes #22.

Developer Testing

Testing done:

• Updated unit tests for generateEventReminderChannelText to handle the manually triggered cases
• Tested functionality on the dev slack, including its handling of error cases when trying to run the command in channels where minerva does not exist.

Reviewer Testing

Steps to view/test

1. Run the /meeting-reminder command in a channel that does not have any upcoming events in the next 24 hours and ensure that you get a message stating such
2. Run the /meeting-reminder command in a channel that has an upcoming event and ensure that a reminder message is sent
3. Run the /meeting-reminder ping command in a channel that has an upcoming event and ensure that a reminder message is sent and it contains and it pings @channel

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This change is 

[shirleyfyx]

Reviewed 9 of 9 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @billyao021031, @jeffrey-huang-yz, @Panizghi, @PCModeActivate, @slightlyskepticalpotat, @wxp02, and @yxyyeah)