Following up on changes in I243f1193bce0da9fa710fc3b5379f90b2d079680 where we
made the talk section overlay pull from the content in the body instead
of delegating to an API.

This cleans up the parts that are obsolete now:

- Removes talkBoard overlay and associated code (talk board is now a
server rendered page instead of an overlay)
-  Removes the talk export in mobile.startup. Minerva is lazy loading
the TalkSectionOverlay/TalkSectionAddOverlay
- Removes `getSections`, `getPage` methods from PageGateway. As far as I can
tell, these were only being used by TalkSectonOverlay/talkBoard  and can
now be removed.
- Removes eventbus from TalkSectionAddOverlay. Minerva (its only client)
is now passing an `onSaveComplete` callback .
- Updates storybook
- Updates tests

Depends-On: I243f1193bce0da9fa710fc3b5379f90b2d079680
Bug: T230695
Change-Id: I75158ff363d56d55ae385687baf64f8b9d5ca8b0