BB 31575: Disable Firefox Home (Activity Stream)

Treat about:blank as the default home page and new tab page.

BB 43886: Disable the newtab component/addon.

Bug 41624: Disable about:pocket-* pages.

Bug 40144: Redirect about:privatebrowsing to the user's home

TB 43886: Drop base-browser changes to AboutNewTabRedirector.sys.mjs
since the module is now excluded.