GUACAMOLE-320: Ensure blank values for the new text attributes of users and connections are properly interpreted.

GUACAMOLE-320: Add parse() convenience function for TextField which interprets empty strings as equivalent to null.

GUACAMOLE-189: Define guacd override columns for guacamole_connection table in main schema init script.

GUACAMOLE-314: Update extension API version sanity check to include the correct, recently-bumped version number.

GUACAMOLE-292: Use ngIf to determine visibility of guacFormField. Using ngShow collides with the ngShow used internally on the guacFormField directive's root element.

GUACAMOLE-292: Restrict attributes on object management pages to those explicitly present on the object.