Restored method for clearing a watchlist in web UI, and rebuilt code.

People have been asking about this, as raw can cause problems for those with
massive lists (upwards of 1 million).

Manually cherry picked from I17dde71858fa65532fbe5d9c2ee2a65a39b2aa87
after short discussion with Brion.

Bug: 13250
Change-Id: I17dde71858fa65532fbe5d9c2ee2a65a39b2aa88