fix(mk,check-privileges): chown/chgrp AFTER user is created
feat(mk,install-infra): condition apt-get update run

Only if last run is older than a day
refactor(mk): add global var for web server user WEB_SERVER
fix(add-channel): solve permissions issues (log files, config)

We can't chown from www-data to mast due to the way permissions work. Also
can't preserve permissions so I stop attempting to change ownership.
* create a 'mast' system group ;
* append 'mast' user to 'www-data' ;
* check-in group user to active permissions

fixes #28