snf_django: Fix escaping of control characters

Fix escaping of control characters from string and unicode objects. The
function 'safe_string' has been renamed to 'escape_ctrl_chars'.
Also, this function has been applied to the output of 'pprint_table'
utility function and has been added as a method of the 'SynnefoCommand'
class.
deploy: Remove '~okeanos' references from settings

Change:
 * 'ASTAKOS_SITENAME' setting value from '~okeanos' to 'Synnefo'
 * 'ASTAKOS_DEFAULT_FROM_EMAIL' setting to 'synnefo_feedback'
docs: Move Archipelago setup to Node 2 prerequisites

Move Archipelago setup to Node 2 prerequisites on install guide.
docs: Update debian install guide

Add Archipelago-imposed limitation to run gunicorn workers as root.