dependencies: upgrade express-session 1.16.1 -> 1.17.0

This upgrade should be backward compatible.
Changelog:
- https://github.com/expressjs/session/blob/master/HISTORY.md

1.17.0 / 2019-10-10
    deps: cookie@0.4.0
        Add SameSite=None support
    deps: safe-buffer@5.2.0

1.16.2 / 2019-06-12
    Fix restoring cookie.originalMaxAge when store returns Date
    deps: parseurl@~1.3.3

src/package.json

@@ -40,7 +40,7 @@
     "etherpad-require-kernel": "1.0.9",
     "etherpad-yajsml": "0.0.2",
     "express": "4.17.1",
-    "express-session": "1.16.1",
+    "express-session": "1.17.0",
     "find-root": "1.1.0",
     "formidable": "1.2.1",
     "graceful-fs": "4.1.15",