core: refactored parsing of config files.
Changed z_sites_manager:get_site_config/1 to return either {ok, Props} or {error, Reason}.
Added z_sites_manager:get_builtin_sites/0.
Don't try to start any sites in priv/sites, unless it is a builtin site.
core: hide the builtin sites from the zotonic_status overview.
mod_oembed: fix for z_string:to_name/1 now returning a binary.
core: remove /priv/sites from the .gitignore.
It now contains sites that are part of the core source tree and should be committed.