Change name of the form and form field for consistency

Following PR review.

Also update function name for update name of security key in
user api client to be more specific.

app/main/forms.py

@@ -2506,8 +2506,8 @@ class BroadcastAreaFormWithSelectAll(BroadcastAreaForm):
         return self.areas.data
 
 
-class ChangeNameOfSecurityKey(StripWhitespaceForm):
-    name_of_key = GovukTextInputField(
+class ChangeSecurityKeyNameForm(StripWhitespaceForm):
+    security_key_name = GovukTextInputField(
         'Name of key',
         validators=[
             DataRequired(message='Cannot be empty'),