Move start user id param to settings

[chrisma]

Currently config.start_user_id = 4000 is set in all three configured environments, i.e. config/environments/production.rb, config/environments/development.rb and config/environments/development.rb.
This is redundant.

Ideally, this setting would be handled in the same way as all other settings, be configurable and have a sane default set in seeds.rb. Alternatively, at least move it to a global config like VmPortal::Application (config/application.rb) to remove the duplication and allow easier maintenance.

See also #399