FIX Add tinymce link menuitems per editor instance

[GuySartorelli]

IMPORTANT NOTE

This PR needs silverstripe/silverstripe-admin#1653 to be merged before CI will go green.

The functionality in silverstripe/silverstripe-admin#1653 is required for this PR to work at all - and the code in this PR will not work with earlier versions of silverstripe/admin. Because of this, it should not be merged until that PR has been merged and tagged, and the new constraint has been checked to ensure it is correct.

Description

Updates the asset-admin TinyMCE link plugin to match silverstripe/silverstripe-admin#1653

Manual testing steps

See issue for reproduction steps

Issues

• Link-dropdown breaks in TinyMCE when other EditorConfig is used on one of the Elements silverstripe-elemental#1120

Pull request checklist

• The target branch is correct
  • See picking the right version
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• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• CI is green

[emteknetnz]

Code in PR is fine, just this needs to be changed

[GuySartorelli]

Done (and done in require instead of require-dev this time - oops)
See https://github.com/silverstripe/silverstripe-admin/releases/tag/1.13.20