Support variables in message bodies #139

Should be able to do things like this for message bodies:

Hey {{first_name}},

Thanks for applying! ...

 - The {{hackathon_name}} Team

Requirements:

• Secure. Can't be such a loose system that it could lead to security holes or syntax that would break the site
• Preview-able. For any supported variable, there should be a standard "example" value that's used while drafting the message.
• Stretch goal: UI for defining custom variables