UpdateReply API and replyUpdateBlocker field for reply

[MrOrz]

As discussed in 20200212 meeting, we want to allow editor to edit their reply when

• The reply is only used by the editor itself (the author should know the impact of the edit)
• The reply has not yet received any feedback (the edit should now invalidate user's feedbacks)

We plan to add 1 mutation API, and one new field to Reply type for this:

"""
The reason why the editor cannot edit
"""
enum ReplyUpdateBlocker {
  NOT_AUTHORIZED # Not logged in or not the author of reply
  HAS_FEEDBACK
  USED_BY_OTHER_EDITORS
}

type Reply {
  """
  The reason why the current user cannot edit this reply. null if the current user can edit.

  The blockers are determined in the following order:
  (1) current user is the author or not
  (2) `positiveFeedbackCount` and `negativeFeedbackCount` are both 0 or not
  (3) all normal article-reply to this article are by the current author or not
  """
  updateBlocker: ReplyUpdateBlocker
}

mutation {
  """
  Updates reply text, type & reference when
  (1) current user is the author
  (2) `positiveFeedbackCount` and `negativeFeedbackCount` are both 0
  (3) all normal article-reply to this article are by the current author
  """
  UpdateReply(
    replyId: String!
    text: String
    type: ReplyTypeEnum
    reference: String
    waitForHyperlinks: Boolean = false
  ) {
    ""'
    The reply after update. `null` when update is not successful
    """
    reply: Reply

    """
    The reason why update fails. null when can update.
    """
    blocker: ReplyUpdateBlocker
  }
}

"all normal article-reply to this article are by the current author" can be achieved by loading article-reply of a reply using articleRepliesByReplyIdLoader and check each article-reply's author ID.

[MrOrz]

As discussed in https://g0v-slack-archive.g0v.ronny.tw/index/channel/C2PPMRQGP/2020-07#ts-1594556835.293700
The return type of the API can leverage union type, as Medium suggested.

[MrOrz]

As discussed in https://g0v.hackmd.io/OC7BneJ3TEGJHge1hWV-0w?both#%E8%A8%8E%E8%AB%96%EF%BC%9A%E8%AE%93%E4%BD%9C%E8%80%85%E8%83%BD%E7%B7%A8%E8%BC%AF%E5%9B%9E%E6%87%89 the priority of this item is set to low.