Add mailgun notifications feature

[enocktito]

No description provided.

[valeriansaliou]

Thanks for the PR! As we already have an email notifier that implements SMTP, would it be rather possible on your end to generate SMTP credentials on Mailgun, and use an SMTP connection rather to send notifications? I'm just wary of implementing multiple notifiers for every API-based emailing provider, that could be nested in a single one (SMTP here).