Move all custom roles code to EE

What does this MR do and why?

• This is an ee-only feature but along the way we accidentally added some of the code to the CE directories. This doesn't mean that custom roles were available to CE users but the code was just in the wrong place.
• The preload logic for the Banzai parser classes can be removed altogether rather than moved to EE because that preloading was necessary for download_code but we changed the custom roles MVC to look at read_code instead (these parsers look at download_code)
• #381611 (closed)

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