Make the sprite editor update faster

[flaki]

• [ ] Ditch the polling on both sides, add websockets.
• [ ] Compress transmitted data
• [ ] Skip session storage for sprite data, store sprite in memory (address by session key)

[flaki]

For transmissions we could use:

• Plain binary ( image_width⨉1_byte_for_every_8_pixels_in_height)
• Plain binary, but XOR-ed changes (should be needed for undo as well)
• Either of above + RLE for transfers.