spamassassin: move into a module

Move the spamassassin manifest & templates into a module. Indent the
manifest, but perform no functional changes at this time.

Change-Id: I52371a77c87a415208f18799260bd43ba638f4a3
spamassassin: cleanup module

This is a separate commit to cleanup the spamassassin module, after
splitting it out. Most of the changes are trivial, the exception being
the OTRS spam scoring that was (and still is) lacking rationale and some
of its settings were removed from being obsolete or not making sense
anymore.

Change-Id: I288bf5c1bd3c22499e00b6449d618c1056de5461