Bug 40416: Add v2 Onion deprecation warnings

• adds new v2 deprecated warning page (js and styling) that piggy-backs off of the existing added onion service errors
• updates identity-icon to onionWarning.svg when visiting a v2 onion site adds warning tooltip; this warning supersedes all other identity states (including mixed-content error)
• backports onionPattern.inc.html and onionPattern.css from 10.5 branch for warning page
• we determine whether to show the warning page in nsDocShell::DoURILoad()
• a new synchonous IPC method is added to ContentChild/ContentParent to determine if the session has loaded the warning page already; worst case scenario, each child process will need to wait on this method to return only once when visiting a v2 onion; nothing is permanently cached with regards to this change
• an exception for the new sync method is added to sync-messages.ini (generally, in practice adding new blocking methods is probably bad, but the minimial overhead and frequency this method is called is worth the simpler code)