The AdminPlugin JS model uses a similar pattern to chat models,
where it is a plain JS class manually converting provided
snake_case attributes from the serializer to JS camelCase.
However this doesn't work when it comes to using `add_to_serializer`
in plugins since core does not know about these new attributes.
Instead, we can use a JS function to convert snake_case to camelCase
and use that when initializing AdminPlugin. This commit also moves
similar functions to a new case-converter.js file in
discourse-common/lib.
