Add the ability to ignore older plans that are no longer part of the current diff

[Fauzyy]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

1. When a user runs atlantis apply, Atlantis should ignore any plan files that are no longer part of the current git diff / pull status.
2. When a user run atlantis apply -p <project_name>, Atlantis should return an error if the plan file is from a project that's no longer part of the current git diff / pull status.

This has come to bite us in the past, where a user runs atlantis apply and it unexpectedly applies workspaces that have since been removed from the git diff. I think we should add a flag which disables running planfiles which are out of scope for the curent PullStatus.

Describe the solution you'd like

To preserve the old behaviour a server-side flag could be added which will ignore plan files that are outside the scope of the current pull status: ignore-stale-plan-files:<true|false>

Describe the drawbacks of your solution

If we're maintaining a server-side flag vs just changing the behaviour here, we'd have to maintain two branches of logic: one for the old behaviour and one for the new.

Describe alternatives you've considered

Instead of a flag we could just change the behaviour of apply to ignore these older plan files, but there should be a stronger consensus from the community for this.

[Fauzyy]

This should be fixed by #1633, I'll close this