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.
Merge pull request #1861 from alphagov/rename_person_philip_marsden_slug

Add a data migration to redirect phillip-marsden to philip-marsden