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Allow to customise addHelpLink() target via system message

Description

Allow to customise addHelpLink() target via system message

Method similar to SpecialPage::outputHeader() to avoid registering
tons of system messages and to have -summary and -helppage tidily
listed together in Special:AllMessages by default.

Bug: T45591
Change-Id: Ic849dde00be7379c1909a8486cf20f48c5aea5cf
(cherry picked from commit e928d5bdd0fbc63a951ca373aaf68bb1e6bf715c)