BootProbes: add a websocket probe

This self-test isn't perfect because we're running it from the backend
instead of the frontend. Conceivably the backend might have trouble
resolving its own url. Eventually we should move this test or
something like it to something that executes in the frontend.

app/client/models/AdminChecks.ts

@@ -163,6 +163,15 @@ It is good practice not to run Grist as the root user.
   'reachable': {
     info: `
 The main page of Grist should be available.
+`
+  },
+
+  'websockets': {
+    // TODO: add a link to https://support.getgrist.com/self-managed/#how-do-i-run-grist-on-a-server
+    info: `
+Websocket connections need HTTP 1.1 and the ability to pass a few
+extra headers in order to work. Sometimes a reverse proxy can
+interfere with these requirements.
 `
   },
 };