Add input_values to Question model

[cortadocodes]

Feature request

Use Case

• Allow input values to be shown for a question model without having to inherit from the deprecated QuestionValuesDatabaseStorageMixin
• Reduce what needs to be added to subclasses of Question so more functionality works out of the box and so the codebase is less complex

Current state

The QuestionValuesDatabaseStorageMixin mixin provides input_values and output_values fields when mixed with Question If we just use the Question model, these fields aren't available and we get an error.

Proposed Solution

Get the input values from an event (possibly by artificially creating one in the ask method but preferably by providing better event support) so input_values can be accessed in the same way as the other question properties in QuestionAdmin (e.g. delivery_acknowledgement)