Wrap spam emails using Swoosh

[adriansalamon]

Changes:

• Wraps API calls to spam with Swoosh, the de-facto standard Elixir email client.
• Removes dependency on Httpoison and replaces it with Req.

This allows us to view emails on /dev/mailbox during development, and send actual emails in production using the same underlying API. Also makes it much simpler if we ever need to swap mail service.