Add change_history to case study API presenter.

This creates some duplication since the top change note within the
change_history hash will be the same as the existing change_note
attribute. However, the latter is already being used by the email alerts
service; it would probably be good to change that app to use
change_history instead.
Timeout of 10s for PDF thumbnail generation

www.agileplannerapp.com/boards/173808/cards/8160
fixes: https://govuk.zendesk.com/tickets/868919

we show the fallback pdf thumbnail image, instead
of a generated one if the thumbnail generation
takes longer than 10s.

better to process and upload without a thumbnail,
than to diasllow it.
Merge pull request #1843 from alphagov/timeout-for-pdf-thumbnail-generation

Timeout of 10s for PDF thumbnail generation
Merge pull request #1857 from alphagov/document_history

Add change_history to case study API presenter.
Add a data migration to redirect phillip-marsden to philip-marsden

- Someone added a ticket to the second-line backlog to rename this
  person's slug due to a typo when it was first created. After a chat
  with Tekin, this was deemed the easiest way. This fixes, in this
  case, the bad spelling of Philip.