Importing telegram stickers with aspect ratio not equal 1:1

[inexcode]

Stickers on Telegram may have one side smaller than 512.
With #250 fixed thumbnails are created correctly now, 512x266 for example, but metadata is incorrect now.

thumbnailWidth: 512,
thumbnailHeight: 512,

Riot for web squeezes stickers with non 1:1 ratio, Riot for Android crops them.
One side is always 512, while another can be anywhere smaller.

I think, the easiest fix is to use one more sharp library method, something like that:

const png = await sharp(buffer).resize({ width: 512, height: 512, fit: 'contain' }).png().toBuffer();

Please note: I didn't test if this works.