HEEDLS-613 Change email change validation to ignore case

[AlexJacksonDS]

JIRA link

https://softwiretech.atlassian.net/browse/HEEDLS-613

Description

Fixed a bug with email validation causing it to reject emails when the case didn't match a user's other account.

Screenshots

N/A

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Developer checks

(Leave tasks unticked if they haven't been appropriate for your ticket.)

I have:

• Run the formatter and made sure there are no IDE errors.
• Written tests for the changes (controller, data services, services, view models etc) and 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.
• 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.

[davidm-m]

Just one thing in the test, otherwise looks good!