Update title and button strings in the new circuit display to sentence case

It looks like the browser chrome is already mostly sentence case (qith the exception of the ETP panel, which we have disabled anyway), so I think we should go ahead and sentence-case-ify the new circuit display straight away.

• Tor Circuit → Tor circuit
• Guard → guard
• New Circuit for this Site → New circuit for this site

^ We actually use New Tor circuit for this site in the application menu already, so we could switch the circuit display to use that string instead for consistency.

Not sure if there are any more strings lurking there.