Use different message when (un)watching a page in talk namespace

Use different message when (un)watching a page in talk namespace

addedwatchtext and removedwatchtext uses the phrase "$1 and its discussion page".
As this is inaccurate on talk namespace pages, two new messages, addedwatchtext-talk
and removedwatchtext-talk, have been introduced for use in talk namespace pages.

Bug: T87856
Change-Id: Iafd86402d3a2a65ce2be007c87c981ac08f44e67