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.

web/webpack.config.ts

@@ -255,11 +255,18 @@ export default (
         name: "server",
         target: "node",
         entry: {
+            katex_server: "babel-loader!./server/katex_server.ts",
             "katex-cli": "shebang-loader!katex/cli",
         },
         output: {
             path: path.resolve(__dirname, "../static/webpack-bundles"),
         },
+        resolve: {
+            alias: {
+                // koa-body uses formidable 2.x, which suffers from https://github.com/node-formidable/formidable/issues/337
+                hexoid: "hexoid/dist/index.js",
+            },
+        },
     };
 
     return [frontendConfig, serverConfig];