Submit responses: ignore outdated responses

[damithc]

Outdated responses (see #6470) cause the submission form of affected students to have To: without student name. In the screenshot below, I've marked where a name will be missing if this situation occurs.

[damithc]

@kanghj I remember you did some investigation to a similar issue before. Any inputs?

[kanghj]

I'll do a more thorough investigation in 1 or 2 days time, but I recall that it may happen if we failed to modify/delete responses due to a change in student's email (or if we are using a task queue for this, the task for modifying the responses hasn't executed).

[kanghj]

The cascaded update to responses for editing emails and teams appears to be correct locally (other than #3404). If this issue is not a frequent occurrence, a possible cause is eventual consistency. Outdated responses will be left if the cascade failed to update all the required responses.

Anyway, we could automatically detect and hide invalid recipients on the feedback submission page as a quick fix.

Linking to #6020 since this is a duplicate

[xpdavid]

In V7:

Under QuestionSubmissionFormMode.FIXED_RECIPIENT mode, outdated responses will be ignore.

Under QuestionSubmissionFormMode.FLEXIBLE_RECIPIENT mode, outdated responses will be used and will cause submission error.