Run migrate script as postinstall task in 6 if the user needs to update their project

[StephenFluin]

Desired behavior

user runs npm install @angular/cli (for v6)

Detect if they need to run ng update @angular/cli --migrate-only --from=1.7.3 and if they do, run it for them! :)

[hansl]

Instead of actually running it, show a message to the user that they need to run it.

[StephenFluin]

Why not run it for them if needed?

[hansl]

Because they might not want that (yet). It's an automated step that can be confusing if it errors out for example.

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