Load only one set of CSS files per module.

There are times, a module is loaded multiple times, and returns same set
of CSS files mulitple times, in order to avoid loading the same file
multiple times, we will create a set out of the list of CSS files.

web/pgadmin/__init__.py

@@ -57,7 +57,7 @@ class PgAdmin(Flask):
         stylesheets = []
         for module in self.submodules:
             stylesheets.extend(getattr(module, "stylesheets", []))
-        return stylesheets
+        return set(stylesheets)
 
     @property
     def messages(self):