Make it impossible to get unsuccessful/incomplete deliveries

[espenak]

Today, students have to first upload their files, and then click Submit delivery. Some students (a very low percent, but it happens), forget to click submit delivery. We should be able to avoid this.

Solution

We assume that students actually want to add any delivery that they have added, so the default should be to mark the delivery as successful. However, we want students to be able to add multiple files, so we can not mark it as successful (which also implies completed) at once. We can solve this as follows:

• We change the successful-attribute to a timestamp named publish_timestamp.

• Each time a student adds a file on a delivery, publish_timestamp is set a couple of minutes in the future.

• After at least one file has been uploaded, the UI shows the student a message with something like this text:

  Your delivery will be submitted automatically in
  [X] minutes and  [Y] seconds unless you add more
  files, or choose to abort the delivery. Click
  *Submit delivery* to submit at once.
  

• The UI must, of course, also show the implied big red Abort delivery and big green Submit delivery buttons.

• All examiner and admin APIs must be updated to filter on now > published_timestamp instead of successful==True

[espenak]

Here is a mockup of how this could look in the UI:

[espenak]

Note that both the deadline and the autosubmit time should be a countdown, not static text.

[tworide]

Default timestamp should then be now() + counter()

[torgeirl]

Solved in #1210 / #1217.