Formatting Improvements for the "Join our mailing list" section at checkout

[kimcoleman]

After our move to table-free checkout page, this Add On was not updated to reflect new core styling, specifically: https://github.com/strangerstudios/pmpro-mailchimp/blob/master/pmpro-mailchimp.php#L495-L508

Also, the "textdomain" should be updated in the translation strings from 'pmpro' to 'pmpro-mailchimp'.

[michaelbeil]

A request for this (ticket 422641 [moderators only):

trying to update the style of the following test provided by the plugin Mailchimp ad on but can’t find were to fix it

[michaelbeil]

PR #127 fixes this.