If there is a syntax error in `settings.py`, `restart-server` should
provide a reasonable message about this. It did so prior to
af08bcdb3f, becausde any invocation `./manage.py` without
`--skip-checks` will verify `settings.py`, among several other checks.
After af08bcdb3f, there are no `./manage.py` calls in most restarts,
which fa77be6e6c took further.
Add an explicit `./manage.py check` in the default case.
upgrade-zulip-stage-2 overrides this by passing `--skip-checks`, for
performance. This also means that `upgrade-zulip-from-git` itself
picks up the same `--skip-checks` flag, since it inherits the same
flag parsing, though that is perhaps of dubious utility.
f5f6a3789b