Replace SpamAssassin with Rspamd

Merged Kristian Klausen requested to merge klausenbusk/infrastructure:rspamd into master
Switching to Rspamd has some advantages:
* It is probably faster than SA[1] (C + Lua vs Perl)
* We can reduce the number of moving parts. Rspamd has built-in DKIM
  signing, greylisting, DMARC checking to name a few
* It doesn't just mark the mail as spam/not-spam, it gives every mail a
  score and depending on the score it does either: nothing, greylist it,
  mark it as spam or reject it[2] (more actions is available and it can
  be tweaked)
* Replies whitelisting[3]
* It supports ARC signing, which can be useful
* A cool looking WebUi :)
* ... and more[4]...


A copy of my own redis/rspamd role for now (slightly modified).


See also:

Edited by Sven-Hendrik Haase

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