Refactor ADVANCED_COMPONENT_TYPES

    Modified to a setting in common.py
    This enables third parties to add XBlocks to their system
    without having to make a code change.
@dcadams aws.py is not like the other settings files, please take a look at some of the examples in it.  It is built so that we can override settings from common.py via json files.  `ADVANCED_COMPONENT_TYPES` should read its value from ENV_TOKENS and default to what is set in common.