The `message.message_reference` stores:

1. `message_id`
2. `channel_id`
3. `guild_id`

We use the `message_id` and `channel_id` to `.findOne(...)` Message, and
find the author. If the author is the same as OP, we skip (so replying
to yourself doesn't add you to the mentions array) otherwise, it adds
the author_id to the mentions array.

Resolves: #1247