implement progress on spawn_pending page

- add Spawner.progress method. Must be an async generator of JSON-able progress events
- add /api/users/:user/server-progress eventstream endpoint
- use eventstream to fill progress bar on the spawn pending page

jupyterhub/handlers/base.py

@@ -887,11 +887,21 @@ class UserSpawnHandler(BaseHandler):
                     pass
 
             # we may have waited above, check pending again:
+            progress_url = url_path_join(
+                self.hub.base_url, 'api/users',
+                user.escaped_name, 'server-progress', spawner.name,
+            )
+
             if spawner.pending:
                 self.log.info("%s is pending %s", spawner._log_name, spawner.pending)
                 # spawn has started, but not finished
                 self.statsd.incr('redirects.user_spawn_pending', 1)
-                html = self.render_template("spawn_pending.html", user=user)
+                url_parts = []
+                html = self.render_template(
+                    "spawn_pending.html",
+                    user=user,
+                    progress_url=progress_url,
+                )
                 self.finish(html)
                 return
 
@@ -919,7 +929,11 @@ class UserSpawnHandler(BaseHandler):
                 self.log.info("%s is pending %s", spawner._log_name, spawner.pending)
                 # spawn has started, but not finished
                 self.statsd.incr('redirects.user_spawn_pending', 1)
-                html = self.render_template("spawn_pending.html", user=user)
+                html = self.render_template(
+                    "spawn_pending.html",
+                    user=user,
+                    progress_url=progress_url,
+                )
                 self.finish(html)
                 return