Merge /api/sendgrid and /api/mailgun

[tomfa]

Endpoints /api/sendgrid/send/<mail> and /api/mailgun/send/<mail> have different behavior

• [ ] sendgrid sends html, mailgun sends plain text. Send html with both?
• [ ] sendgrid html mail does not contain the same text as its text version.
• [ ] sendgrid allows for setting subject and text via GET param. Remove and use content that mailgun uses?
• [ ] Extract sendgrid and mailgun code to a library, and replace api endpoints with /api/email/login/<email>?

[rix1]

After thinking briefly I'm implementing the following endpoint auth-now.sh/api/email/