This lets roundcube's manageseive plugin do cool things like vacation responses.

Also:

* Run the spam filtering sieve script out of a global sieve file that we'll place in /etc/dovecot. It is no longer necessary to create per-user sieve files for this. Remove them with a new migration. Remove the code that created them.

* Corrects the spam script. Backslashes were double-escaped probably because this script started embedded within the bash script. Not sure how this was working until now.

this adapts work by @h8h in #103

As the scripts keep growing, it's time to split them up to
keep them understandable.

This splits mail.sh into mail-postfix.sh, mail-dovecot.sh,
and mail-users.sh, which has all of the user database-related
configurations shared by Dovecot and Postfix. Also from
spamassassin.sh the core sieve configuration is moved into
mail-dovecot.sh and the virtual transport setting is moved
into mail-postfix.sh.

Also revising one of the sed scripts in mail-dovecot to
not insert a new additional # at the start of a line each
time the script is run.

closes #94

in cf7053c1 I allowed editconf.py to insert a setting where we find it already commented-out in order to get an nginx configuration line in the right place, but it wasn't quite right because when run again we would insert the setting a second time

the new migration management in c8856f10 left out the part where we actually keep the system's current MIGRATIONID... it was being lost when setup/start.sh was re-run

whats_next: Allow the PRIMARY_HOSTNAME to not have an MX because the default value means the domain itself, which is what we want anyway

clarify that an SSL cert can remain self-signed on the non-primary domains if the domain isn't being used for web

migrate the SSL certificates path for non-primary certs to a new layout using a new migration script

move the SSH password login check into whats_next.py (it used to be in start.sh and then moved to an unused script when it became a problem for Vagrant)

Moved the configuration to a single YAML file, rather than one per domain, to be clearer.

re-does 33f06f29