Merge2: Assignment state flow

• assignment states (and manual progressing through states)
  • For assigning reviews (#117 (closed)) and marking reviews as done (#166 (closed))
• Add flexible deadline for submissions (so before the reviews are assigned) #173 (closed)
• Add flexible deadline for reviews (so students can still submit their reviews in the feedback phase, but not unsubmit them) #129 (closed)

NOTE: changes here affect MR 169 as well, so either MR should be adapted, depending on which is merged first