Actually update the code of the main app so that users can set values for inequalities and have them recorded into their parameters json files ready to be sent to the model.

Steps:

1. Adapt the mock-up into the app (include description and rationale boxes as well)
2. Make the UI dynamic depending on the scheme/site (different schemes will have different HRG codes available to them).
3. Write the selected values to the params json.
4. Update the JSON schema.

Prerequisites:

• https://github.com/The-Strategy-Unit/demand_modelling/issues/25
• Create mockup of suggested inequalities design #398 (mock-up has been added to nhp_analysis)
• Create inequalities text description #403
• Add inequalities into params #476

This will require UI and server modules (and most likely a utils module) to be written.

We must ensure that the data are written to the params in the form given by #379 (comment).