core: Fix issue with m_edge trying to insert NULL values for seq
core: Use a standard global zotonic.config file

We now use a standard zotonic.config file, which is formatted just like
application config files in Erlang. This file is read by Erlang on VM
startup. Writing config file is no longer supported (z_config:set/2) as
this was only used to set the global password. A password is now
generated by bash when the zotonic.config file does not yet exist.

Fixes #403

See also #767