Clean up k-diffusion sigma params

- the setting for preferring opts.sd_vae has been inverted and reworded
- resolve_vae function made easier to read and now returns an object rather than a tuple
- if the checkbox for overriding per-model preferences is checked, opts.sd_vae overrides checkpoint user metadata
- changing VAE in user metadata  for currently loaded model immediately applies the selection

add explicit content-type header for image/webp

Add missing k-diffusion sigma params to infotext

Gradio theme cache

Allow `s_noise` override to actually be used

don't save incomplete images

Update README.md

Increase `s_churn` max value

Add option for `s_tmax`

Correct install instructions on linux and provide additional required apt packages
Fixes #12351

add new gradio themes