GeezerVison

We previously checked that the content was below GitHub's 25M limit, but
this was done in the request handler. `aiohttp` _already_ checks the
content size and has a limit of 1 MiB.

Instead, set the limit for `aiohttp` and for Caddy directly. Though the
latter is redundant, it's possibly a bit more secure. Limiting upload to
the regular site is also probably redundant since it goes to
`file_server` which supports no uploads, but better to cut that off
early. CloudFlare also has a limit set, but it's to its minimum allowed
which is 100MB.