[jupyter] configure also ServerApp to let use static with jupyter lab

Also add test for JSROOTSYS to let use special JSROOT local installation
as it THttpServer does

etc/notebook/jupyter_notebook_config.py.in

 import os
-if 'ROOTSYS' in os.environ:
-    # Prefer using JSROOT from ROOTSYS if defined
+if 'JSROOTSYS' in os.environ:
+    # Let use localy installed JSROOT as THttpServer does
+    c.NotebookApp.extra_static_paths.append(os.environ['JSROOTSYS'])
+    c.ServerApp.extra_static_paths.append(os.environ['JSROOTSYS'])
+elif 'ROOTSYS' in os.environ:
+    # By default use JSROOT from ROOTSYS if defined
     c.NotebookApp.extra_static_paths.append(os.path.join(os.environ['ROOTSYS'], 'js/'))
+    c.ServerApp.extra_static_paths.append(os.path.join(os.environ['ROOTSYS'], 'js/'))
 else:
     # Fall back to CMAKE_INSTALL_PREFIX/CMAKE_INSTALL_JSROOTDIR, e.g., for a system installation
     c.NotebookApp.extra_static_paths.append(os.path.join("@CMAKE_INSTALL_PREFIX@", "@CMAKE_INSTALL_JSROOTDIR@"))
+    c.ServerApp.extra_static_paths.append(os.path.join("@CMAKE_INSTALL_PREFIX@", "@CMAKE_INSTALL_JSROOTDIR@"))