Copy recommendedPhaseTargetDate while in same candidate review window for a test plan version

[howard-e]

Address latest comments in #973 (comment).

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[gnarf]

I'm not familiar enough with the workflow to provide a good local test that it is now working correctly, but the code seems correct and the additional comments about the window are useful.

[alflennik]

This was a doozy to test! With some tweaking of my system clock I was able to verify that the target dates are preserved when deprecating a candidate test plan, and that the target dates are NOT preserved when there is no candidate test plan.