Remove special Watchlist star handling on Flow boards

Notice the slightly hacky way to still rely on MW's default watch star handling
(I didn't want to throw that away for consistency - don't want to go
reimplement triggered events, spinning stars, ...) but show our own tooltip
instead.

This patch simply adjusts the message within the standard mw notification
when watchlisting the board.  It replaces the default message with our
own.

Change-Id: I3f31544d83ee60f11454bdd8d4a8f31e508bd706
Merge "Remove special Watchlist star handling on Flow boards"