Reactions of deleted message still persist in database

[Antreesy]

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Steps to reproduce

1. Post a message
2. Add a reaction to it
3. Delete a message

Expected behaviour

Server response should be:

{
  "id": 122,
  "message": "Message deleted by you",
  "messageType": "comment_deleted",
  "reactions": {},
},
{
  "id": 123,
  "message": "Reaction deleted by author",
  "parent": {
    "id": 122,
  }
},
{
  "id": 124,
  "message": "You deleted a message",
  "systemMessage": "message_deleted",
  "parent": {
    "id": 122,
  }
},

Actual behaviour

{
  "id": 122,
  "message": "Message deleted by you",
  "messageType": "comment_deleted",
  "reactions": {},
  "reactionsSelf": [
    "\ud83d\udc4d"
  ]
},
{
  "id": 123,
  "message": "\ud83d\udc4d",
  "systemMessage": "reaction",
  "parent": {
    "id": 122,
  }
},
{
  "id": 124,
  "message": "You deleted a message",
  "systemMessage": "message_deleted",
  "parent": {
    "id": 122,
  }
},

[SystemKeeper]

Similar issue with message expiration at #11298

[nickvergessen]

Will have a look, but not sure this is easily fixable and worth it.
If we have to cascade the deletion, deleting a message could result in many hidden system messages, even worse on expiration where we don't even track which messages got deleted

[nickvergessen]

If a single "message deleted" system message is good enough, we could add a single query like:

DELETE FROM oc_comments WHERE (verb = 'reaction' OR verb = 'reaction_deleted') AND parent_id = X;

However the system message of a reaction_revoked is much harder to delete, as it would be verb = 'system' AND message = '{"message":"reaction_revoked","parameters":{"message":REACTION_MESSAGE_ID}}' so requiring a list of IDs before hitting the query and potentially having a huge result, or we simply delete all system messages that are "child" to the deleted message. That would delete edit traces etc also for the future, so not sure we'd like that.