This patch:
- adds an 'unsubscribetoken' column to usr_watchlist_view table
 (if we want to unsubscribe other messages we'd need to add an
'unsubscribetoken' to the relevant table)
- records the token when one watches the view
- sends email with unsubscribe link with message type and token to
avoid exposing any user data
- link goes to unsubscribe.php page and either unsubscribes user or
gives generic warning

Currently only working for watchlist notifications being sent via
email/email digest

Change-Id: I823249108f521faaefe3435f03b84ddf73e2d360
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>