Automate RAM disk configuration for spool directory

[amotl]

Hi there,

in order to reduce wear on the SD card when running Saraswati on SBC-like computer systems, the spool directory /var/spool/saraswati should be able to be configured as RAM disk, see [1].

It would be sweet to make those configuration steps available under the saraswati setup subcommand.

With kind regards,
Andreas.

[1] https://github.com/hiveeyes/saraswati/blob/0.6.0/doc/setup-production.rst#using-a-ram-disk