[WIP] Add Mooclet Policy Parameters JSON Editor to Design Step

[zackcl]

This PR adds the "Mooclet Policy Parameters" JSON editor below the "Payloads" table in the "Design" step. I've moved the existing JSON editor styles from the "Add Metrics" modal (add-metrics.component.scss and add-metrics.theme.scss) to styles.scss so that the styles can be applied globally.

TODO:

• Make the "Mooclet Policy Parameters" JSON editor visible only when the "Assignment Algorithm" isn't "Random" or "Stratified Random Sampling"
• Add mock policy parameter interfaces (schemas) and the default values to display in the JSON editor
• Fetch all available policies, schemas, and default parameters from the backend either on startup or stepper open
• Implement validation of the user-edited JSON object

Related Issues: #2074, #1937