DLSV2-559 Fixes validation for supervisor enrolment

[kevwhitt-hee]

JIRA link

DLSV2-559

Description

Adds validation to supervisor enrol delegate on self assessment / choose supervisor role page. Uses standard validation implementation with view components.

Screenshots

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Developer checks

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I have:

• Run the formatter and made sure there are no IDE errors.
• 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. Full manual testing guidelines can be found here: https://softwiretech.atlassian.net/wiki/spaces/HEE/pages/6703648740/Testing
• Updated/added documentation in Swiki and/or Readme. Links (if any) are below:
  • 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 own MR to ensure everything is as expected.