Set min/maxversion for plugins to STD_VERSION unless otherwise specified

Currently, a missing min/max version will ALWAYS cause a failure of the version comparisons for themes and addons.

To avoid the need to set non-sensical values (like a max version that doesn't yet exist) or updating theme/addon files with EVERY new Hubzilla version, we set missing values to the current version - essentially resulting in missing values being ignored - which seems to be the anticipated result of the older array_key_exists checks done in lines 658ff.