Move 'show subscriber name' setting from block config to mailing list config

Issue Summary

This is more-or-less a followup to:

#531794: Admin UI for individual emails + list import feature

Per discussion with litwol, this patch does the following:

• Moves the 'show subscriber name' checkbox from the block config for a mailing list's subscription form, to the 'edit list' form of a mailing list
• Creates a new field in the 'mailing_list' DB table, called 'show_subscriber_name'
• Migrates the old 'mailing_list_show_name_x' variables to the new DB table field (in a new update.php function)
• Changes wording in the UI from 'rename list' to 'edit list'