Alex/add latex compiler key

Add latex_compiler_key to course settings.

True - will enable problem with latex templates.
False - will disable.

Problem or Html templates, which use latex, should have latex in their names.

Fix for  https://edx-wiki.atlassian.net/browse/BLD-426

Special notes:
a) When course author open course (default value for use_latex_compiler is false), he does not see latex problems. Then he changes value to true. And if he will press back, that page will not reload and he will se old templates for advanced problems. He should reload page to see new advanced templates. Html templates are not affected.

@frrrances and @singingwolfboy please review