v3: split /api/list into /api/files and /api/shortens

• split the files blueprint into list and delete blueprints
• replace the dictionaries returned in /api/list to ordered lists of objects
  • pro: the client doesn't need to deal with sorting the data itsel
  • con: checking for existence of a file (in our tests) is now a linear operation instead of constant

cc @a @Mstrodl

• remove pagination, use before/after
• add tests for before/after