Add nested flash link check

[darkdh]

fix #8581

Auditors: @diracdeltas, @bbondy

Test Plan:
Covered by automatic test

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

[darkdh]

@diracdeltas , there is still one problem with this PR. If I go to https://play.hbogo.com/ and click the flash link, preventDefault() doesn't constrain new tab from being spawned.