Slack plugin and fixing some potential mail issues

[slaschinski]

The major edit is that I added the Slack plugin to front- and backend. Then I added a bit to the e-mail config. That fixed some issues for me. Even though my mail provider uses TLS and I used the port for TLS, I'm still getting timeouts when I say "EMAIL_USE_TLS" instead of "EMAIL_USE_SSL". There are some issues on github describing similar problems and solutions.