Notification is not sent when a comment is added in a sub-section of the topic one subscribed to.

This happens because the interface only allows you to subscribe to sections with level 2 headings, but we attempt to send notifications to subscribers of the closest heading, even if it's level 3 (or some other level).

Test case: https://en.wikipedia.beta.wmflabs.org/wiki/Talk:T286736

For example, this problem would occur in discussions like this:

• …when another section has been merged into this one as a sub-section:
  • https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_190#Font_sizes_all_over_the_place
  • https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_190#Large_font_for_categories_in_articles
• …when an arbitrary section break is added, so that commenters don't have to scroll so far to click [edit source]:
  • https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(proposals)/Archive_166#Request_for_comment_on_the_future_of_Wikipedia:In_the_news
  • https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(proposals)/Archive_166#(arbitrary_break)