TD-4327-Supervisor delegate validation added

[Auldrin-Possa]

JIRA link

TD-4327

Description

Code added to verify if supervisor delegate exists.
Used existing error page, as there is 'Forbidden' error page is being used for 403 status code in the application.
Note - For centres with learner role only, an Access Denied page will be displayed.

Screenshots

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Developer checks

I have:

• Run the formatter and made sure there are no IDE errors (see info on Text Editor settings to avoid whitespace changes)
• Written tests for the changes (accessibility tests, unit tests for controller, data services, services, view models, etc)
• Manually tested my work with and without JavaScript
• Tested any Views or partials created or changed with Wave Chrome plugin and addressed any valid accessibility issues
• Updated/added documentation in Confluence and/or GitHub Readme. List of documentation links added/changed:
  • doc_1_here
• Updated my Jira ticket with information about other parts of the system that were touched as part of the MR and have to be sanity tested to ensure nothing’s broken
• Scanned over my pull request in GitHub and addressed any warnings from the GitHub Build and Test checks.