WIP: Add collapsible submissions view - #1560

• Currently there is a "unread" notification, to have this work across browser sessions we would have to save it on the back end, so:
  • Do we want this feature? (I would like to have it as it allows better overview on already processed responses)

Unread flag would be nice, this could also be represented in the list of forms on the left

• One global "viewed" flag per submission or one per user? (I would prefer it per user)

I would say that it's enough to have it per submission, but per user would be nice

• Saved within the submission? Or in the share? Or a new table? Somewhere else?

Depends on the point above... If per submission, then with the submission. If per user, probably somewhere else

• I implemented this currently as a separate view, should we keep this as a third view or replace the "responses" view with this?

A separate view isn't necessary in my opinion... We're showing the same information as in the responses view. So in conjunction with the "unread" flag you could for example collapse all read submissions by default and have a collapse/expand all action on top.

@susnux As this PR is quite old, should we keep it open?