Support for dynamic locale

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There are places in this sim where dynamic locale is not currently supported. Any place where you're getting StringProperty.value will not automatically update when the user changes locale, or when tested with ?stringTest=dynamic.

There are some that are straightforward to identify, like this:

// GreenhouseConcentrationPanel.ts
    const iceAgeLabel = GreenhouseEffectStrings.concentrationPanel.iceAgeStringProperty.value;

The others appear to be related to description, search for "StringProperty.value". Do those need to be dynamic?

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In the above commit, I fixed a problem related to iceAgeStringProperty, and factored out constants for years ('2020', etc.)

The remaining cases where I see StringProperty used in a way that is not dynamic all appear to be related to description. @jbphet does description need to dynamically update when locale changes? If so, there is a lot more work to do here. If not, this issue can be closed.

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@jbphet and I discussed this. Since there's currently no way to translate descriptions, there's currently no need to make the implementation handle dynamic locale. And doing so might be a big effort. So for now we should just be sure that dynamic locale and description features interact "well" together -- we'll discuss that in #299.

Closing.