Add warning banner for download table preview for entities with lots of vars

[asizemore]

Resolves #1652

Current behavior

Now it looks like:

That error banner only comes up once i've clicked at least one var.

Is it weird to have both banners? I thought about removing the server-error banner, but i dont want to hide errors that might be useful to us to see later...

[dmfalke]

I think we want to avoid making the request if showPreviewTableWarning === true.

I would rename that variable to something like canLoadTablePreview and define it closer to the top of the component, and use that value as a part of this condition.

[dmfalke]

I would rename that variable to something like canLoadTablePreview and define it closer to the top of the component, and use that value as a part of this condition.

Hmm... maybe this would be a better place to add the check.

[asizemore]

Thanks @dmfalke ! Now after the update it looks like: