[Connection Settings] Point Help buttons to the offline manual

Now that we merged the offline manual, we should point Help buttons to it.

• Existing URLs in TorStrings
• about:preferences#connection (and use TorStrings where not using it!)
  • "Select a Built-In Bridge..." help button to Circumvention
  • "Request a Bridge..." to Bridges
  • "Add a Bridge Manually..." to Bridges
• about:preferences#privacy
  • Onion location
  • Security level
• Onion location notification (to test: go to about:config and set privacy.prioritizeonions.showNotification to true)
• Security level panel
• about:tor
• Bookmarks