Merge different delete activities for better compatibility (fixes #2066)

[Nutomic]

Lemmy has Delete.object like this:

  "object": {
    "id": "http://ds9.lemmy.ml/post/1",
    "type": "Tombstone"
  },

While Pleroma uses:

  "object": "https://greenish.red/objects/38e2b983-ebf5-4387-9bc2-3b80305469c9",

Our approach doesnt make much sense, because all we really care about is the object id. For now i'm making a backwards compatible change to accept both types of deletes, but for 0.16 we should remove the compat code and do it just like Pleroma.

[Nutomic]

Added a second commit which makes the breaking change to remove the compatibility code. Dont squash this, the first commit should be cherry-picked into 0.15.3, while the second commit should be released with 0.16.0

Edit: I think the test failure is because Delete and DeletePrivateMessage have the exact same structure now, so the json can get parsed into the wrong struct. To solve this we will have to merge both structs into one, which means that this will require a bigger rewrite.