Revise and define flexible and great paste visibility strategies

[jamo]

Currently we have only two pastebin visibility strategies, neither one is really good.

Investigate new relevant strategies, based on research interests, usability and user experience, and best possible peer support (students helping each other).

[jamo]

We need to refactor strategies to be exercise specific.
As we may have exercises where the only test is assetTrue(true) and people may still need to ask help for that specific exercise.

[jamo]

And have a strategy allwaysVisibleHideAfterXhours