Cleanup: TemplateContentModelImpl needs a lot of attention

[dhughes-xumak]

TemplateContentModelImpl has several issues which will make it difficult for external contributors to work with it. Examples:

• General organization. There are public, private, protected, and default visibility methods scattered throughout the class. There is also an enum and a field defined in seemingly random locations between methods. Perhaps these could be reordered to promote readability and clarity.
• Methods with default visibility (package-private) are a code smell. If possible, let's get rid of these.
• Some methods are synchronized, but the class as a whole is not thread-safe. Do these methods need to be synchronized? Could a comment be added to provide some explanation?
• Some fields are volatile, but I don't think this provides the thread-safety that they were intended to. Reconsider these.
• There are unused methods such as newInstance(). There is also commented out code. Remove these.
• There are unnecessary methods which should be removed, such as response() (is only used internally by wrappedResponse(), and simply returns a field) and toJSONString() (used only by toString()). Likewise, there is a non-private inner class CharResponseWrapper which is only used once, internally. That could be inlined as an anonymous inner class.
• Generally, I don't think wrappedResponse() belongs in this class. The response wrapping should probably be moved to the calling class: IncludeResourceHelperFunction.

[dhughes-xumak]

label:"help wanted"

[josecarlosgt]

The workaround for solving the issues with the wrappedResponse method is to:

1. Move the method wrappedResponse to a static method in the class IncludeResourceHelperFunction. This will also include moving the functionality of the inner class CharResponseWrapper.
2. Make the class CharResponseWrapper anonymous in the method mentioned above.

@jbarrera-xumak please add any comment if necessary.

[josecarlosgt]

Solved in #36