v3: email rewrite and webhook revamp

• use smtp instead of mailgun api
  • MAILGUN_* configuration fields are now replaced by SMTP_CONFIG
  • move of all email related functions to api.common.email, just like all webhook functions are in api.common.webhook
  • the use of smtplib runs in an executor since it could take a while
  • email functions don't use app parameters and instead use Quart.current_app
  • enhancement: make smtp related errors go into our own EmailError instead of returning false
  • make a move and make all email and webhook functions raise errors, instead of returning false
• all webhook functions check if the webhook is properly declared and correctly fail
• misc: some functions from the datadump background task had their app parameter removed (still need to finish, but i did just enough to get to the email part)