svc: Save config after navigation

When a makes a *POST /screen/{name}/navigate* HTTP request, the URL list
is updated and saved to the configuration file.  This provides a
mechanism for configuring the URL list, since it is now part of the
display configuration file and not a separate file manged externally.

svc/src/hudctrl/api.py

@@ -137,8 +137,14 @@ async def get_screenshot(
     '/screen/{name}/navigate',
     response_class=fastapi.responses.PlainTextResponse,
 )
-async def navigate(name: str, url: str = fastapi.Form(...)):
+async def navigate(
+    bgtasks: fastapi.BackgroundTasks,
+    name: str,
+    url: str = fastapi.Form(...),
+):
     await svc.navigate(name, url)
+    svc.urls[name] = url
+    bgtasks.add_task(svc.save_config)
 
 
 @app.on_event('shutdown')