FIX Correctly validate emails in WYSIWYG links

[GuySartorelli]

Description

Ensure the correct EmailField form field is used for entering email addresses in WYSIWYG links. This provides appropriate validation errors when incorrect values are entered.

Note I haven't added behat for this because shockingly there is no behat at all in this module for WYSIWYG links - so adding them in this PR for this fix is a bit overkill.

Manual testing steps

In any WYSIWYG field, try to add an email address link with an invalid email address. You should be given an appropriate validation error within the modal form.

Issues

• https://github.com/silverstripe/silverstripe-admin/issues/1707

Pull request checklist

• [x] The target branch is correct
  • See picking the right version
• [x] All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• [x] The commit messages follow our commit message guidelines
• [x] The PR follows our contribution guidelines
• [x] Code changes follow our coding conventions
• [x] This change is covered with tests (or tests aren't necessary for this change)
• [ ] CI is green

[GuySartorelli]

JS CI failure is unrelated to the changes in this PR.