[Loops] Improve user experience regarding subscriptions to mailing list

## Problem

There are a good number of frustrated users because either:
1. They never signed up to receive Hack Club emails
2. or have a hard time unsubscribing/opting out.

## Proposed Solution

After chatting with Zach, we will:

1. Inform users that they'll receive Hack Club-wide communication. Something along the lines of
> We are a charity and as part of this, you'll get emails about the rest of Hack Club's free programs so you can get involved :smile:

This would be visible on the user settings page and should be visible during sign-up and normal user editing.
2. Provide a way for people to unsubscribe/opt out. By default, we will continue to opt in to maximize the number of people subscribed to Hack Club announcements. However, we MUST provide an easy way for users to unsubscribe & opt-out directly within HCB. At the moment, users have to receive at least one email from Hack Club, then click the unsubscribe link within the email— that's a bad experience.