[SAC] TracSpamFilter disabled

[Sandro Santilli]

Due to recent frustration of new users (GSoC students) I've disabled
the TracSpamFilter plugin.

Problem were mostly:

 - Legit content considered spam by external services
 - Legit content considered spam by bayesian filter

Given the effort we make to accept new users, I've disabled it
alltogheter.

NOTE: configuration cannot be changed from the web at the moment,
      because all trac.ini are not writeable by the webserver user
      (you'd get a forever-spinning browser request)

Shall spam come back, head on to /etc/trac/ and revert the
git commit which disabled the plugin:

   git revert 4186dbbac626aedfb058e462a70389ad8f42aa64

I'd love to make a second round of fixes/upgrades to trac but
have too many things to do at the moment.

--strk;