Add getBreakageFormOptions message handler

[kzar]

The privacy-dashboard is going to be sending a new message
"getBreakageFormOptions", which is very similar to "getToggleReportOptions"
except for use by the standard breakage report UI flow. Let's add that now,
taking care to share the logic between those message handlers.

Reviewer: @jdorweiler, @mgurgel

Steps to test this PR:

1. Disable protections for a website, then click to see report details.

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
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