Back End: Reminder and post-donation emails

[shawnadelic]

I'll probably tackle this task, but essentially we need to:

• Implement templates
• Add management command for sending both
• Management command can be automated with Heroku scheduled task

[ndanielsen]

@shawnadelic can you give me a sense of the models to involved for the Reminder and Post-donation logic?

I can probably tackle this next Tuesday (if it isn't complete by then)