Use knowls to show hints and solutions

The following configurations (in defaults.config) control behavior:

+# Show solution button
+$pg{options}{show_solution_button} = 0;
+
+# Show hint button
+$pg{options}{show_hint_button} = 0;
+
+# Use knowls for hints
+$pg{specialPGEnvironmentVars}{use_knowls_for_hints} = 1;
+
+# Use knowls for solutions
+$pg{specialPGEnvironmentVars}{use_knowls_for_solutions} = 1;
+