[Task]: Change 'newsletter' to 'mailing list' in all email templates

[alexis-bunc]

Summary

After the Simpler.Grants.gov website has been updated to say 'mailing list' instead of 'newsletter,' change each individual email template to use the same language.

Note that this issue can't be closed until the website itself is changed.

Acceptance criteria

Each individual email template has been changed from 'newsletter' to 'mailing list' and the link has been changed to reflect the new URL (if applicable).