[SAC] Global trac config

I've modified the configuration of all trac instances to inherit
configurations from the /etc/trac/trac-common.ini file, and updated
the wiki page mentioning this global configuration to use when
creating new instances [1]

The local trac.ini, which is the only one that can be tweaked via
web by admins, overrides the global configuration.

I've put the global configuration under a local git repository.
At the moment it only enables logging (to /var/log/trac/trac_all.log)
and the spam filter plugin. In the future we may want to add more.

For the sake of reducing entropy, I wonder if we should also disable
the web configuration interface and _only_ allow configuring from
the shell. This idea is also backed by recent experiences in getting
"locked out" of the web interface on some onexpected error (for
example enabling the spam filter plugin which required a database
schema upgrade or setting the log file to a non-writeable directory).

[1] Trac - OSGeo