[6.0] Fix regression where external links are rendering as plaintext

[mportiz08]

• Explanation: Fixes regression where external links render as plaintext
• Scope: Impacts pages with links to external sources
• Issue: rdar://133444687
• Risk: Low, focused change to make JS logic more strict when stripping link data
• Testing: Updated unit tests, manually tested pages with internal and external link content, verified that known external links are still purposefully rendered as plaintext in the scenario from Inactivate known external links #878
• Reviewer: @hqhhuang
• Original PR: Fix regression where external links are rendering as plaintext #886

[mportiz08]

@swift-ci test