Add project validation command: check

[evansd]

Proposed name: opensafely check

This should run the project validation code from jobrunner. As we add further kinds of checks e.g. for codelists or study definitions it should run those too. Ideally it would be a one stop "is everything OK with my project command".

We might want to support warnings as well as outright errors. For example, having uncommitted changes should probably be a warning.

[bloodearnest]

We have added a check command, but it check dataset usage, not project vaildation.

Now were using t]e pipeline library, this fairly simple.