show warning message and scary icon on data: urls

[diracdeltas]

to prevent phishing attempts.
fix #4899

Test Plan:

1. automated tests should pass
2. enter data:text/html,hi in the urlbar
3. you should see a popup warning you about phishing

• 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).

Test Plan:

[darkdh]

lgtm