[FIX][IMP] portal, sale: fix and improve UX for signature form in customer portal

 * set height for draw signature when user want to accept order to avoid
   various display issues;
 * fix name input not being displayed as error if trying to sign with a
   void partner name;
 * no more show 'Accept & Sign' button if customer signed document;
 * update message string for success message after signature;
 * add an option to specify the signature button label and use it in
   sale customer portal;
 * improve return of signature route to return more specific error
   messages;

This commit contains squashed code from @rop-odoo .