[LDAT-256] Sign in flow

[danielburnley]

Context

The build in amplify ui components don't conform to the NHS Design system - we needed to ensure we had a flexible login UI to handle when designs change.

This is the first of a few PRs to migrate this.

Changes proposed in this pull request

• Uses slightly more secure login methods for cognito
• Removes amplify ui components for login
• Reimplements the CognitoLogin component with custom logic
  • Currently supports:
  • Initial login & initial password setting
  • Logging in existing users
• Add authentication API interface & cognito implementaion

Guidance to review

• Check out the branch
• Create a new user in cognito
  • Assign it a group
• Login, create your new password
• Logout
• Login again

Link to Jira task

https://bluesquirrel.atlassian.net/browse/LDAT-256