Blur image previews

[Sheraff]

compute 10x10 blurred base64 preview of images (at the same time when we compute the palette) and use it as a background on <img> tags

Requires

• npm run db to add column in Image table
• a call to /api/reset-palettes to compute the previews

Not merged yet because

• might want to calibrate the creation of blurs a little (try a few ≠ sizes, try a specific image format, ...)
• it adds a pretty big column to Image that gets returned by many procedures
• might want to try using a div behind the img instead of the img itself for applying the background so it doesn't flash to no-background when the image file is loaded but not decoded yet