Extract logic from template.

Instead of looking up available indexes in the template and sorting
them, now there is a method in ZCatalog.

modified:   src/Products/ZCatalog/ZCatalog.py
modified:   src/Products/ZCatalog/zpt/catalogIndexes.zpt

src/Products/ZCatalog/ZCatalog.py

@@ -862,6 +862,17 @@ def addIndex(self, name, type, extra=None):
 
         self._catalog.addIndex(name, index)
 
+    @security.protected(manage_zcatalog_indexes)
+    def availableIndexes(self):
+        """Return a sorted list of indexes.
+
+        Only indexes get returned for which the user has adequate
+        permission to add them.
+        """
+        return sorted(
+            self.Indexes.filtered_meta_types(),
+            key=lambda meta_types: meta_types['name'])
+
     @security.protected(manage_zcatalog_indexes)
     def delIndex(self, name):
         self._catalog.delIndex(name)

src/Products/ZCatalog/zpt/catalogIndexes.zpt

@@ -12,7 +12,7 @@
 		added index. You may want to update the whole Catalog.
 	</p>
 	<tal:indexes define="indexes context/Indexes">
-		<tal:add define="filtered_meta_types python:sorted(indexes.filtered_meta_types(), key=lambda mt: mt['name'])">
+		<tal:add define="filtered_meta_types context/availableIndexes">
 			<form tal:attributes="action context/absolute_url" method="get">
 				<div class="form-group mt-4 mb-4">
 					<div class="input-group">