Add Peak Month aggregation function.

[Aritra8438]

Fixes #3005

This PR adds Peak Month aggregation function to the Mathesar UI.

Technical details

I have done the following things:

• Add custom PostgreSQL function for Peak Month aggregation.
• Add Peak Month to the Mathesar UI.
• Add tests for the same.

Screenshots

Checklist

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  visible errors.

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[mathemancer]

Big picture is the same as my previous review. I think the calculations should be less discrete, and open the potential for figuring out the peak day in a year, or peak month, or peak week (Germans would like this).

[seancolsen]

Front end changes look good.

All my concerns from Peak Day of Week apply here as well though.

[mathemancer]

Looks great overall! Thanks for the great comments explaining the functions.

The only change I have to request is to remove the casting of output months to strings. They should either be integers, or 2-digit formatted numeric strings (e.g., '01', '02', '11', '12')

[mathemancer]

Okay, this looks good. Thank you!