Adds an assets server to the demo worker, and reworks the existing asset
server to use the same code. There are a few simplifications to the code
due to some DX improvements working with R2 and caches. I also removed
the `HEAD` request from the assets server: i took a look at our logs and
it's not actually used at all.
This also fixes an issue where users could overwrite the contents of the
asset uploads bucket.
### Change type
- [x] `other`
dcfc6da604