Log giphy usage and opening of websockets

Tests can now use --throw-deprecation (since express 5), experiment w…

Improve compression of single-channel images

Fix styling of bullet points in security page

Bump supported nodejs versions

Run prettier (one-off) on all files

Improve giphy UI, use alt text if available

Remove some module mocking in frontend tests

Replace less with raw css now that browsers have moved on

Bump dependencies, notably patch regex-dos attack in express