Add API action=watch 'entirewatchlist' param to bulk-clear watchlist.

Requested by bot runners, who often accumulate huge watchlists
and have trouble resetting them.

Requires POST and a token to prevent accidents or abuse.
Same token as individual or list watching.

Bug: 65058
Change-Id: Id8476a1ba8b2661cf6451cacf3121d5c2d0675cf
Merge "Add API action=watch 'entirewatchlist' param to bulk-clear watchlist."