Addressing issue #464 (#757)

- Always displaying the displayName isn't a option as a custom field (_requireDisplayName) has been created specificially for this
- The email address on the signin and signup controllers have been disabled from editing for the following reason: the email address is being verified in the previous screen and allowing the user to edit it at this point bypass all the previous validations (so the user can potentially signup with an existing email address for example)

Author: SebastienFCT

EmailAuth/FirebaseEmailAuthUI/FUIPasswordSignInViewController.m

@@ -249,6 +249,7 @@ - (UITableViewCell *)tableView:(UITableView *)tableView
   cell.textField.delegate = self;
   if (indexPath.row == 0) {
     cell.label.text = FUILocalizedString(kStr_Email);
+    cell.textField.enabled = NO;
     _emailField = cell.textField;
     _emailField.text = _email;
     _emailField.placeholder = FUILocalizedString(kStr_EnterYourEmail);

EmailAuth/FirebaseEmailAuthUI/FUIPasswordSignUpViewController.m

@@ -271,6 +271,7 @@ - (UITableViewCell *)tableView:(UITableView *)tableView
   if (indexPath.row == 0) {
     cell.label.text = FUILocalizedString(kStr_Email);
     cell.accessibilityIdentifier = kEmailSignUpCellAccessibilityID;
+    cell.textField.enabled = NO;
     _emailField = cell.textField;
     _emailField.text = _email;
     _emailField.placeholder = FUILocalizedString(kStr_EnterYourEmail);