Add method to clear mako template lookups

Quick and dirty fix (edxapp.startup.run is called twice, once before
loading the configuration, so the paths defined in lms.startup are
basically ignored).

common/djangoapps/edxmako/__init__.py

@@ -13,4 +13,4 @@
 #   limitations under the License.
 LOOKUP = {}
 
-from .paths import add_lookup, lookup_template
+from .paths import add_lookup, lookup_template, clear_lookups

common/djangoapps/edxmako/paths.py

@@ -22,6 +22,13 @@ class DynamicTemplateLookup(TemplateLookup):
         self.directories.append(os.path.normpath(directory))
 
 
+def clear_lookups(namespace):
+    """
+    Remove mako template lookups for the given namespace.
+    """
+    if namespace in LOOKUP:
+        del LOOKUP[namespace]
+
 def add_lookup(namespace, directory, package=None):
     """
     Adds a new mako template lookup directory to the given namespace.

common/djangoapps/edxmako/startup.py

@@ -2,7 +2,7 @@
 Initialize the mako template lookup
 """
 from django.conf import settings
-from . import add_lookup
+from . import add_lookup, clear_lookups
 
 
 def run():
@@ -14,5 +14,6 @@ def run():
     """
     template_locations = settings.MAKO_TEMPLATES
     for namespace, directories in template_locations.items():
+        clear_lookups(namespace)
         for directory in directories:
             add_lookup(namespace, directory)