Modify the plugin_helper to handle zero_to_one record types.

Currently it assumes that plugin-added record types will always be
arrays.  Allow it to be a single value (or nil) too.
Fix to allow plugins to extension JSON schemas when loaded.

The implementation there was using a relative directory without being
explicit about what it was relative to.  This worked for plugins that
are bundled with ArchivesSpace, but not for plugins that were added to
the distribution after the fact.