katex: Replace subprocess call with minimal external service.

Replace a separate call to subprocess, starting `node` from scratch,
with an optional standalone node Express service which performs the
rendering.  In benchmarking, this reduces the overhead of a KaTeX call
from 120ms to 2.8ms.  This is notable because enough calls to KaTeX in
a single message would previously time out the whole message
rendering.

The service is optional because he majority of deployments do not use
enough LaTeX to merit the additional memory usage (60Mb).

Fixes: #17425.

tools/webpack

@@ -46,7 +46,7 @@ def build_for_dev_server(host: str, port: str, minify: bool, disable_host_check:
     webpack_args = ["../node_modules/.bin/webpack-cli", "serve"]
     webpack_args += [
         # webpack-cli has a bug where it ignores --watch-poll with
-        # multi-config, and we don't need the katex-cli part anyway.
+        # multi-config, and we don't need the katex part anyway.
         "--config-name=frontend",
         f"--host={host}",
         f"--port={port}",