[SAC] trac spamfilter global config

I just found out the spamfilter configuration is set in
the trac.ini file, so I've added recommended settings in
a global /etc/trac/spamfilter.ini

In order for the global setting to be used, there must be
NO override of the settings in the local trac.ini

This isn't easy as whenever you "Apply changes" in the admin
panel you'll be saving locally whatever was set at the time
of form submission.

Anyway for now I *removed* the [smap-filter] section from all
local trac.ini, so that all instances have the same global
settings. I hope this will simplify management.

You'll still need per-instance bayes training and BadContent
wiki page.