When "viewing" exports, the self-user chat was displayed as <external> due to
flawed logic of except that would fail to assign the "self" user to the
conversation.user field.

Gory details

The dm chat entry in export's dms.json would have a slice of users
participating in DM, such as:

[
  {
    "id": "DHYNUJ00Y", // conversation ID
    "created": 1555493778,
    "members": [       // participants
      "UHSD97ZA5",     // this is you.
      "UABCDEFGH"      // this is someone else.
    ]
  }
]

To determine the user ID to assign to a DM conversation, the logic would:

1. try to determine current user IDs by scanning through all DMs
   and finding the "most frequent user" ID.
2. Then, to assign the users to other DMs, it would pick the first user in the
   slice, that is NOT the user discovered on step 1.

For chats with others, it would correctly return the User ID of the "other"
participant. For example, for the payload in the beginning of this section, it
would return "UABCDEFGH"

The chat with "yourself" looks like this in the export's dms.json:

[
  {
    "id": "DHYNUJ00Y",
    "created": 1555493778,
    "members": [
      "UHSD97ZA5",
      "UHSD97ZA5"
    ]
  }
]

• Step 1 would discover that the most frequent user is UHSD97ZA5.
• Step 2 would exclude this user and return an empty string.

As a result, the conversation would have an empty User ID and would not display
properly in the viewer.

Thank you, you're welcome, come again.