Add RepoBee 4 review commands as preview feature

[slarse]

The review commands are to be restructured according to the discussion here #790.

Essentially, reviews-assign remains more or less unchanged, but it spits out a review allocation file, which is then used by reviews check and reviews end. So, calling e.g. reviews check should be as simple as:

$ repobee reviews check --allocations-file allocations.json

• Add initial support for RepoBee 4 versions of review commands
• Verify support for double-blind peer review (may or may not work out of the box)
• Verify support for group reviews (probably won't work out of the box)
  • This is of course if we even want to keep this, perhaps reviews should simply be individual work