Rearchitect our implementation so that about:tor pages are always
loaded in a content process. This also fixes:
  Bug 22535: Searching brings me to duckduckgo but my query is discarded.
  Bug 21948: Going back to about:tor page gives "Address isn't valid" error.

Most of the code that initializes and updates about:tor content has
been moved to a content script. When necessary, IPC is used to pass
data from the chrome process to the content script.

Removed old, no-longer-used m_tb_orig_BrowserOnAboutPageLoad variable
from torbutton.js.

Also, update the about:tor newChannel() implementation to accept an
nsILoadInfo parameter.