Minor bug when restarting server with custom tags

This patch has limited testing w.r.t. whether the bug is completely
gone, but it appears to have no negative side effects, and fix at
least one case.

courseware/modules/__init__.py

@@ -33,16 +33,24 @@ from courseware import content_parser
 modx_module_list = [capa_module,  html_module,  schematic_module,  seq_module,  template_module,  vertical_module,  video_module]
 #print modx_module_list
 
-# Convert list to a dictionary for lookup by tag
 modx_modules = {}
-for module in modx_module_list:
-    for tag in module.Module.get_xml_tags():
-        modx_modules[tag] = module.Module
+
+# Convert list to a dictionary for lookup by tag
+def update_modules():
+    global modx_modules
+    modx_modules = dict()
+    for module in modx_module_list:
+        for tag in module.Module.get_xml_tags():
+            modx_modules[tag] = module.Module
+
+update_modules()
 
 def get_module_class(tag):
     ''' Given an XML tag (e.g. 'video'), return 
     the associated module (e.g. video_module.Module). 
     '''
+    if tag not in modx_modules:
+        update_modules()
     return modx_modules[tag]
 
 def get_module_id(tag):