Adds a configuration flag for opt-in ladders to always show Thank You page

[StoneMoney]

Requested and Used in client themes for RAN and SPCAI

• Displays the "Thank You" page of the opt-in ladder in the event the supporter returns to a page that contains the opt-in ladder while already opted in to all prior steps.
• Set data-engrid-opt-in-ladder-persist=true on the iframe/config url
• Demonstration link: https://donate.spcai.org/page/185332/subscriptions/1?mode=DEMO

Testing instructions:

• Testing new behavior: Visit demonstration link
• Enter the email of a blank/new supporter
• Submit the form
• Complete all steps of the opt-in ladder, the embedded thank you page contains social media links/icons
• Close the page, and reopen the demonstration link
• Enter any email you wish to use for testing
• Submit the form with "Action Alerts and General Updates" and "SMS Text Alerts" checked.
• On the thank you page, the embedded thank you page of the opt in ladder (social media links) should appear, since you've already opted into general updates and sms text alerts.
• Testing default behavior: Enter page builder, and remove data-engrid-opt-in-ladder-persist=true in the opt-in ladder code block on page 2 and save
• Visit demonstration link
• Enter any email you wish to use for testing
• Submit the form with "Action Alerts and General Updates" and "SMS Text Alerts" checked.
• The thank you page should not contain social media icons.