Po files allow translation of untranslatable strings

The po parser plugin is currently generating translation strings for all strings in the json files, including the 'id' and 'control' strings[1].

This breaks the pages.

The po parser should not give the possibility of translating those.

[1] https://gitweb.torproject.org/translation.git/tree/support-miscellaneous.json?h=support-miscellaneous