Mention: notify correctly if there is no section title

If a user is mentioned in an area that does not have a section
title (for example, the lead section of a talk page), use the new
'notification-mention-nosection' message, to avoid an awkward and
incorrect "[[Page#|]]".

Bug: 52507
Change-Id: I40628b76c460c79008053222c9780bb23549731d
Merge "Mention: notify correctly if there is no section title"