Bug 23247: Communicating security expectations for .onion

Encrypting pages hosted on Onion Services with SSL/TLS is redundant
(in terms of hiding content) as all traffic within the Tor network is
already fully encrypted.  Therefore, serving HTTP pages from an Onion
Service is more or less fine.

Prior to this patch, Tor Browser would mostly treat pages delivered
via Onion Services as well as pages delivered in the ordinary fashion
over the internet in the same way.  This created some inconsistencies
in behaviour and misinformation presented to the user relating to the
security of pages delivered via Onion Services:

 - HTTP Onion Service pages did not have any 'lock' icon indicating
   the site was secure
 - HTTP Onion Service pages would be marked as unencrypted in the Page
   Info screen
 - Mixed-mode content restrictions did not apply to HTTP Onion Service
   pages embedding Non-Onion HTTP content

This patch fixes the above issues, and also adds several new 'Onion'
icons to the mix to indicate all of the various permutations of Onion
Services hosted HTTP or HTTPS pages with HTTP or HTTPS content.

Strings for Onion Service Page Info page are pulled from Torbutton's
localization strings.