redesign: render thumbnails at a larger font size

[marcelgerber]

@sophiamersmann, if you could take a look at 0a31261 for a moment that would be great.

Basically, the rationale is that if a title spans two or more lines, then we benefit from the font size reduction even if that doesn't reduce the number of lines.
I also changed the font size reduction to a maximum of 20% rather than cap it at 2px. It's just another arbitrary number, so feel free to change.

In theory we could also make this behavior dependent on the isGeneratingThumbnail flag, so let me know what you think.

Here are some thumbnails rendered using the worker. The last three have a reduced title font size.

[sophiamersmann]

Looks good to me!

For "normal" static charts, the range would now be 24px -> ~20px which is perfectly fine, I think. Removing the additional check if reducing the font size didn't help also makes sense to me – I put it in with low confidence.