Move ProjectRequest input validation to ProjectDescriptionCustomizer

[snicoll]

Currently, DefaultProjectRequestToDescriptionConverter does the conversion from ProjectRequest to ProjectDescription and clean up invalid values or provide defaults in case of missing entries.

#1023 offers a way to track those changes but that won't track anything that is changed upfront. We should move all that cleaning logic to a proper ProjectDescriptionCustomizer so that we're able to see the value was changed.

Note that a side effect of that is that a bunch of properties will be flagged as changed if no original value was provided. This sounds like an added bonus to me as we'll be able to track if the user has provided no value vs. an invalid value we had to clean.