Trac #19373: Jupyter help links do not work in Jupyterhub

When running Sage's Jupyter notebook within a Jupyterhub environment
(probably also other kinds of proxies), the help links using
`/kernelspecs` do not work. Instead, a relative link `../kernelspecs`
does work.

URL: http://trac.sagemath.org/19373
Reported by: jdemeyer
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun

src/sage/repl/ipython_kernel/kernel.py

@@ -87,12 +87,12 @@ def help_links(self):
             sage: sk = SageKernel.__new__(SageKernel)
             sage: sk.help_links
             [{'text': 'Sage Documentation',
-              'url': '/kernelspecs/sage_.../doc/index.html'},
+              'url': '../kernelspecs/sage_.../doc/index.html'},
              ...]
         """
         from sage.repl.ipython_kernel.install import SageKernelSpec
         identifier = SageKernelSpec.identifier()
-        kernel_url = lambda x: '/kernelspecs/{0}/{1}'.format(identifier, x)
+        kernel_url = lambda x: '../kernelspecs/{0}/{1}'.format(identifier, x)
         return [
             {
                 'text': 'Sage Documentation',
@@ -119,7 +119,7 @@ def help_links(self):
                 'url': kernel_url('doc/reference/index.html'),
             },
             {
-                'text': 'Developers Guide',
+                'text': "Developer's Guide",
                 'url': kernel_url('doc/developer/index.html'),
             },
             {