Restore nav link underlines in Forced Colors mode

[chris-lawton]

In Forced Colors mode, the top-level nav links and the dropdown links weren't underlined.

• Top-level links: a text-decoration: none placed after the high-contrast-text-decoration mixin was overriding its underline.
• Dropdown row and card links: these never included the mixin.

The fix gives the mixin an optional $remove-underline: true argument. With it, the mixin removes the default underline before adding the forced-colors one, so the order is always right and stylelint still passes.

To test: turn on Forced Colors mode and check that the top-level nav and dropdown links are underlined. Outside Forced Colors mode, they should look the same as before.

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[jhancock532]

Tested locally, LGTM!