300 streamline frontend refresh network calls

[pmarsh-scottlogic]

Description

When you first load the website in the browser, or refresh we only send one network request which should grab all the necessary information to get going.

Notes

• Creates GET /start endpoint which is the single network call the frontend needs to set itself up. It returns the available models, the system roles for all levels and the emails, history and defences for the current level.
• The above network call acts as the frontend's health check so I've removed the healthcheck stuff from the frontend
• removes GET /openai/validModels endpoint and all related code (no longer needed)
• removes GET /systemRoles endpoint and all related code (no longer needed)
• moves loadBackendData and openHandbook to mainComponent.tsx so that all the /start fetched state stuff can be managed by one component.
• Adds UseIsFirstRender hook, for using with the useEffect hook. It indicates whether the useEffect is running on the first mount or on the change of a dependency.

Checklist

Have you done the following?

• [x] Linked the relevant Issue
• [ ] Added tests
• [x] Ensured the workflow steps are passing

[chriswilty]

@pmarsh-scottlogic Reviewing shortly, lunch first...