[Recurring Donation] Disable `DonationMailer#donation_notification`. Replace with monthly summary email

[garyhtou]

The DonationMailer#donation_notification gets quite repetitive for monthly donations.

Let's disable them. However, we should still send them for the 1st round of a recurring donation.

Then, at the end of each month, we'll send a monthly summary email of all donations (including recurring donations)

[garyhtou]

https://github.com/hackclub/hcb/pull/8407

[garyhtou]

Sarvesh is interested in taking this on!

[garyhtou]

Hey Sarvesh, I'm going to assign this to @polypixeldev

[garyhtou]

I'd recommend reading this guide about Mailers. Focus on sections 1, 2, 4 (first two paragraphs only), 5, 9, and skim 7.

As a first step, you’ll want to create a mailer called DonationMailer with an action called monthly_summary. It should probably take in a date parameter called month and an event (org). Then, the mailer will list all the one-off donations and recurring donations from that month, and summarize it in an email