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/common/BootProbe.ts

@@ -7,7 +7,8 @@ export type BootProbeIds =
     'host-header' |
     'sandboxing' |
     'system-user' |
-    'authentication'
+    'authentication' |
+    'websockets'
 ;
 
 export interface BootProbeResult {