Restrict max. body size

[dmke]

From my TODO list:

• [x] wrap the /render handler in a MaxBytesReader
• [x] add a --max-render-size CLI flag

[dmke]

The flag is now --max-job-size, as other flags are named similar (--job-directory, --parallel-jobs).

[dmke]

CI failure is related to https://github.com/actions/setup-go/issues/206 (Go version 1.x still points to 1.17.x, http.MaxBytesHandler is a Go 1.18 feature).