context_processors.py: Add flag for whether page is help center.

This is needed once the header markup is deduplicated.

zerver/lib/test_helpers.py

@@ -271,6 +271,7 @@ class POSTRequestMock(object):
         self._tornado_handler = DummyHandler()
         self._log_data = {}  # type: Dict[str, Any]
         self.META = {'PATH_INFO': 'test'}
+        self.path = ''
 
 class HostRequestMock(object):
     """A mock request object where get_host() works.  Useful for testing
@@ -282,6 +283,7 @@ class HostRequestMock(object):
         self.GET = {}  # type: Dict[str, Any]
         self.POST = {}  # type: Dict[str, Any]
         self.META = {'PATH_INFO': 'test'}
+        self.path = ''
 
     def get_host(self):
         # type: () -> Text