Provide PLG_LIST_ENVELOPE_ENVELOPE string

For dropdown menu in list, this plugin asks for
PLG_LIST_ENVELOPE_ENVELOPE which does not exist.

This commit provides this as the same string as PLG_LIST_EMAIL_EMAIL
which seems to be in error (but which is retained in case it is used by
anyone in eval code).

P.S. Error reported in http://fabrikar.com/forums/index.php?threads/email-list-plugin-translation-missing.35960/ and si applicable to both F3.0 and F3.1.