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).
Merge pull request #985 from Sophist-UK/fix_list_plugin_email_string

Provide PLG_LIST_ENVELOPE_ENVELOPE string