Replace velocity-animate with something less prone to problems

[turt2live]

velocity-animate causes a number of problems for us regularly, leaving holes in read receipts and generally being unkind to us. It's largely unmaintained at this point as well: as of writing, it's gone 2 years without a stable release and 1 year without a beta. The author seems to be working on it as a hobby, but in the meantime CSS animations have grown to be more and more powerful.

We should look into some other solution to both reduce bundle size but also have a more stable/predictable implementation for our read receipt waterfall.

See matrix-org/matrix-react-sdk#5697 and matrix-org/matrix-react-sdk#3688 for links to related issues.