4176 customizing reminder message to partners

[nathangthomas]

Resolves rubyforgood#4176

Description

This PR allows a custom message to be added to the distribution reminder message similar to how we currently make customizations on the distribution email content.

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Tested manually
• Wrote two tests in the distribution_mailer_spec that account for a distribution that includes a custom_reminder

Provide instructions so we can reproduce.
You can test the email manually by:

• run the app locally bin/start
• go to http://localhost:3000/diaper_bank/distributions/1/edit
• add a custom message and ensure that the distribution date and time is for a date in the future.
• open rails c
• run DistributionMailer.reminder_email(Distribution.first.id).deliver_now A mailer preview should open in your browser.

Screenshots

[cielf]

Hey @nathangthomas -- I see you have two prs related to this, and the second one is WIP. Are you looking to have this one merged, or the second one? Thanks.