We have to move the activation logic from a ve.ui.Command to the
ve.ui.Tool, as the Command is unable to refer to the Target.
Previous attempt: 4984c5ffbb
reverted in fb32aa4978.
Bug: T279313
Change-Id: I3cb74aa5123b67a6c63b8e07ea7f93a6d4a07d4f
This would would be correct if it was in a method of ve.ui.MWBackTool,
but this is actually a different class ve.ui.MWBackCommand (they are
both in the same file), which doesn't have a 'toolbar' property.
This partially reverts commit 4984c5ffbb
"Avoid using mw.ArticleTarget methods on global ve.init.target in tools".
Bug: T279613
Change-Id: Ia5e80e34cc5021295639cf18b3c324d2821aecf5
