Score thumbnails are stretched and quality is too low for high-res screen

Describe the bug
In Home > Scores, the score thumbnails are stretched, especially when the paper size is very different than the shape of the thumbnail rectangle. For example, here is one of the VTests. The score looks like this:

And the thumbnail looks like this:

Additionally, on high-res screens, the thumbnail image looks blurry, because the resolution of the image is not high enough.

Expected behavior
That's to be decided by the designers (@Tantacrul, @jessjwilliamson)

Possible solution
For the stretching problem: simply adjust some QML code.
For the resolution problem: the only solution is to store the images at a higher resolution in the mscz files. This will only fix it for new scores, not for existing ones (until you save them in MU4).

Additional information
Thumbnails for imported files have no problems, since they go through a different process.