Set mammograms/ and participants/ active nav component

These url namespaces currently have 'Home' active in the nav when
navigating to their pages. Set it to 'Screening' instead.

Author: malcolmbaig

manage_breast_screening/core/context_processors.py

@@ -1,5 +1,5 @@
 def nav_active(request):
-    if request.path.startswith("/clinics/"):
+    if request.path.startswith(("/clinics/", "/mammograms/", "/participants/")):
         active = "clinics"
     else:
         active = "home"