Fix the (un)watch token to include the namespace name.

Title::getDBkey() only returns the page name without the namespace
which means that "Test" and "User:Test" (for example) pages would
have the same token; use Title::getPrefixedDBkey() instead to
avoid this.

Change-Id: I80333b23cec0cfe6546f6e7776b0a77b56ee20c8
Merge "Fix the (un)watch token to include the namespace name."