Address initial review comments from #94

[nhnt11]

1. Put breach list API endpoint URL in a pref
2. Define _delayedInit as a property of FirefoxMonitor and document it
3. Use Preferences.reset to invalidate warned hosts cache pref
4. Fix nits
5. Put breach list refresh timeout in a pref
6. Add comment about isLoadingDocument
7. Add comment explaining win.gBrowser check in EveryWindow unregister callback
8. Resolve TODOs in loadStrings
9. Call removeAddonListener as part of cleanup
10. Set textContent property where possible instead of creating text node and appending
11. Use IDs instead of classes where appropriate
12. Ship custom warning.svg to support dark theme and for stability
13. Comment about font-size style.

[nhnt11]

All done!