"Learn more" button is not shown on mobile talk pages to display content defined in MediaWiki:Talkpageheader.

Affected wikis: https://global-search.toolforge.org/?q=.&regex=1&namespaces=8&title=Talkpageheader

Test plan

Button should appear on these pages instead of the Talkpageheader message, and reveal it on click:

• https://zh.m.wikipedia.org/wiki/Talk:復仇者聯盟：秘密戰爭?dtenable=1 (has Talkpageheader, page doesn't exist)
• https://hr.m.wikipedia.org/wiki/Razgovor:Gracia_(glazbeni_sastav)?dtenable=1 (has Talkpageheader, no normal lede section, page exists)
• https://fr.m.wikipedia.org/wiki/Discussion:IC_3253?dtenable=1 (has Talkpageheader and normal lede section)

Button should not appear on these pages:

• https://frr.m.wikipedia.org/wiki/Diskussion:Viborg?dtenable=1 (has Talkpageheader, but it's empty)
• https://pl.m.wikipedia.org/wiki/Dyskusja:Shinsengumi?dtenable=1 (no Talkpageheader or lede)