Changing how the enotifusertalkpages user option is handled by Echo

If we're using Echo to handle user talk page post notifications,
hide the old (non-Echo) preference for this. If Echo is moved to core
we'll want to remove this old user option entirely. For now, though,
we need to keep it defined in case Echo is ever uninstalled.
Otherwise, that preference could be lost entirely. This hiding logic
is not abstracted since there is only a single preference in core
that is potentially made obsolete by Echo.

Change-Id: I6c8b2143a5fd719486f7e6cbaa3d7635f507ffca
Merge "Changing how the enotifusertalkpages user option is handled by Echo"