- Update the API index definitions used by the tests for the `api` service
   to be in sync with the actual index returned by https://hypothes.is/api/.

 - Move the definitions into a separate file to make them easier to update in
   future and add instructions on how to update them.

 - Fix two errors in the tests; use of "PUT" instead of "PATCH" for
   updating an annotation and an incorrect parameter name in the API call
   to remove a group member.