Email notifications on new comments for partners

[wes-otf]

Fixes #3871. Allows partners that are assigned to an application to also get email notifications when a new comment is made that has applicable visibility. There was some refactoring that took place to allow for email messages to be customized on a per-recipient basis rather than having one be generated for all recipients.

Test Steps

• [ ] Assign one or more partners to an application
• [ ] As staff, comment on the application with the visibility of Partners
• [ ] Ensure only partners receive the notification email
• [ ] As staff, comment on the application with the visibility of Applicants
• [ ] Ensure only the applicant receives the notification email
• [ ] As staff, comment on the application with the visibility of Applicants & Partners
• [ ] Ensure both the applicant & partners receive the notification email
• [ ] Try some of the same logic but become partner and/or applicant and ensure email notifications have a similar result as the previous steps

[wes-otf]

thanks for testing y'all! all the feedback is fantastic @theskumar

[wes-otf]

this should be all set, had a few kinks but added more unit tests to ensure they don't come back

[wes-otf]

can we get this in? staff are pinging me saying they're having a hard time getting replies from partners without reaching out to them