[feat] Add initial implementation of RepoBee 4 review commands

[slarse]

#867

This is an initial, and pretty shaky, implementation of the RepoBee 4 review commands. If the environment variable REPOBEE_4_REVIEW_COMMANDS is set to true, then these new review commands replace the existing ones. So, in a "normal" setup, the new review commands are invisible. This PR also introduces some neat infrastructure for handling feature flags.

Given that the environment variable is set appropriately, the following applies:

• reviews assign is unchanged in terms of usage, but it now spits out a review_allocations.json file.
• reviews check takes only two arguments (in addition to the base arguments): --allocations-file and --title-regex
  • --allocations-file takes the provided allocations file.
  • --title-regex works as before, and will very likely be removed altogether in future iterations.
• reviews end only takes the --allocations-file argument (in addition to base arguments)

This is just the first draft of this functionality. Group reviews don't work properly, and there may be some kinks with double-blind as well. The details will change, but this represents the general idea of the simplification of the interface.

[codecov]

Codecov Report

Merging #868 (0540707) into master (672532e) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #868      +/-   ##
==========================================
+ Coverage   90.60%   90.79%   +0.18%     
==========================================
  Files          70       71       +1     
  Lines        3726     3779      +53     
  Branches      546      556      +10     
==========================================
+ Hits         3376     3431      +55     
+ Misses        281      280       -1     
+ Partials       69       68       -1     

Impacted Files	Coverage Δ
src/_repobee/cli/dispatch.py	87.91% <100.00%> (+0.55%)	⬆️
src/_repobee/cli/mainparser.py	100.00% <100.00%> (ø)
src/_repobee/command/peer.py	96.71% <100.00%> (+0.67%)	⬆️
src/_repobee/featflags.py	100.00% <100.00%> (ø)
src/repobee_plug/config.py	89.36% <0.00%> (+0.72%)	⬆️
src/_repobee/cli/argparse_ext.py	92.98% <0.00%> (+3.50%)	⬆️

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