Fix exposure of dm field in item documents

[hategan]

Before #15 tests were failing because /dm/session/:id1/item/:id2 was moved to /dm/fs/item/:id. Both endpoints were supposed to return item dictionaries with the 'dm' field included. I believe that the former worked based on the understanding that tests were passing at some time. @Xarthisius reports that /dm/fs/item/:id did not expose the 'dm' field in items and opted for adding the 'dm' field to the list of exposed fields for the normal item API, while switching the tests to simply use /item/:id.

The fact that /dm/session/:id1/item/:id2 was only called from the tests means that exposing the 'dm' field in all instances makes it useless in most instances. This issue is here to remind us that we might opt for a different solution in the future.