default oauth token expiry to cookie_max_age_days

so changing cookie age changes oauth token expiry,
since these are what are stored in those cookies anyway,
it makes sense for them to expire at the same time

jupyterhub/app.py

@@ -358,9 +358,11 @@ class JupyterHub(Application):
     ).tag(config=True)
 
     oauth_token_expires_in = Integer(
-        24 * 3600,
         help="""Expiry (in seconds) of OAuth access tokens.
 
+        The default is to expire when the cookie storing them expires,
+        according to `cookie_max_age_days` config.
+
         These are the tokens stored in cookies when you visit
         a single-user server or service.
         When they expire, you must re-authenticate with the Hub,
@@ -376,12 +378,20 @@ class JupyterHub(Application):
         .. versionadded:: 1.4
             OAuth token expires_in was not previously configurable.
         .. versionchanged:: 1.4
-            Default is now one day.
+            Default now uses cookie_max_age_days so that oauth tokens
+            which are generally stored in cookies,
+            expire when the cookies storing them expire.
             Previously, it was one hour.
         """,
         config=True,
     )
 
+    @default("oauth_token_expires_in")
+    def _cookie_max_age_seconds(self):
+        """default to cookie max age, where these tokens are stored"""
+        # convert cookie max age days to seconds
+        return int(self.cookie_max_age_days * 24 * 3600)
+
     redirect_to_server = Bool(
         True, help="Redirect user to server (if running), instead of control panel."
     ).tag(config=True)