Further HCM fixes and update style lint packages to cope with system colours (sort of)

[helenb]

Link to Ticket

Description of Changes Made

Adds link underline, and thicker underline on hover, to navigation items in HCM
Does the same for rich text links in HCM
Makes the 'More' button consistent with the HCM version in the careers site

How to Test

To test in high contrast mode, ensure 'forced colours: active' has been selected in dev tools under 'rendering'. Then adjust the 'prefers-color-scheme' option to light or dark and test both modes.

Test both the home page and an inner page. To test rich text links, edit a standard page and add a paragraph block and add a link to it.

Screenshots

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MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Consider adding unit tests, especially for bug fixes. If you don't, tell us why.
• Tests and linting passes.
• Consider updating documentation. If you don't, tell us why.
• If relevant, list the environments / browsers in which you tested your changes: Chrome plus dev tools to mimic HCM