Deploy admin form and sub centre page UI improvements
This MR brings several tweaks and improvements as per discussion between @john.twyman and @seamus. Notably:
- makes Header, Footer and Thank you fields in the admin form optional rather than mandatory
- improves the presentation of these fields' labels in the admin form
- updates sub centre Page template to handle the possibility the header and footer fields are empty
- resolves a deprecation warning in
CRM_Emailsubscriptioncentre_Utils::getDisplayName()
while also improving how to handle cases where contacts have incomplete/"weird" names - updates README.md to contain enough information for people familiar with the work to get started on using the extension
Tested locally and in our dev environment.