Because of the non-restartless nature of Torbutton, it required
a two-stage installation process. On mobile, it was a problem,
because it was not loading when the user opened the browser for
the first time.

Moving it to tor-browser and making it a system extension allows it
to load when the user opens the browser for first time.

Additionally, this patch also fixes Bug 27611.

Bug 26321: New Circuit and New Identity menu items

Bug 14392: Make about:tor behave like other initial pages.

Bug 25013: Add torbutton as a tor-browser submodule

Bug 31575: Replace Firefox Home (newtab) with about:tor

Avoid loading AboutNewTab in BrowserGlue.jsm in order
to avoid several network requests that we do not need. Besides,
about:newtab will now point to about:blank or about:tor (depending
on browser.newtabpage.enabled) and about:home will point to
about:tor.