Unwatch link for pages in Special:Watchlist

Unwatch link for pages in Special:Watchlist

When the 'watchlistunwatchlinks' preference option is enabled, this
adds a '×' link to each entry of the watchlist that unwatches the page
of that entry. When clicked, it changes into a '+' which can be used to
re-watch the page (effectively undoing the earlier unwatch).
When a page is unwatched, its entries and the entries of its associated
talk page (or vice versa) become translucent and are struck through.

Without JS, '×'/'+' link to action=(un)watch for the relevant page.

In addition, ChangesList classes have been modified to allow a prefixer
that adds a prefix to each line (used in this case to put the unwatch
link) and to add HTML data attributes to reliably determine the target
page of each entry. Unit tests have been updated accordingly.

Bug: T2424
Change-Id: I450b2901413d7e75c11de2a446829fdbb22d31e1