Replace SpamAssassin with Rspamd

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]...

[1] https://rspamd.com/doc/tutorials/migrate_sa.html#why-migrate-to-rspamd
[2] https://rspamd.com/doc/faq.html#what-are-rspamd-actions
[3] https://rspamd.com/doc/modules/replies.html
[4] https://rspamd.com/comparison.html

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

Why?

• Rspamd is faster according to some sources
  • It scales better (event-driven vs pre-fork)
• We can reduce the number of moving parts. Rspamd can replace OpenDKIM, Postgrey and probably also Postfwd
• DMARC support
• ARC support (could be useful for the mailing list)
• Replies whitelisting
• WebUi

See also: https://rspamd.com/comparison.html