Decouple application from external services

[franklindyer]

The app should be decoupled from some of the external APIs upon which it relies, namely Mailgun and reCaptcha.

Ideally, the following things should be changed:

• The AgoraEmailer should be made into an interface, of which the Mailgun emailer is only one implementation
• Other emailers should be available, e.g. one that uses Gmail, or one that uses Discord or text messaging
• It should be possible to run the application without using captchas at all
• Captcha usage should be abstracted through an interface, so that services other than reCaptcha can be used
• The database manager should support other SQL DBMSs