Reactions aren't transferred when changing usernames or deleting accounts

Description:

Once you change your username or delete the account, the reactions aren't transferred/removed. This can turn out to be a significant flaw for its usage as a polling mechanism or even new accounts having the remnant reactions of an old/changed account.

The reactions are stored in the message objects in a very raw format. If you :heart_eyes: react something, it is stored as
"reactions" : { ":heart_eyes:" : { "usernames" : [ username ] }}
This makes querying to find reactions by username infeasible.
It would be better to migrate to a structure like:
"reactions" : [ { "type": ":heart_eyes:", "usernames": [ username ] }]
This might take a bit more space in the document but allows manipulation of the reactions by usernames.

Steps to reproduce:

Expected behavior:

For the reactions to be transferred/deleted.

Actual behavior:

The reactions aren't changed.

Server Setup Information:

• Version of Rocket.Chat Server: 3.11.0-develop
• Operating System: Ubuntu 20.10
• Deployment Method: develop
• Number of Running Instances: 1
• DB Replicaset Oplog: Enabled
• NodeJS Version: 12.18.4 - x64
• MongoDB Version: 4.2.8

Client Setup Information

• Desktop App or Browser Version: Chrome Version 87.0.4280.88
• Operating System: Ubuntu 20.10

Additional context

Relevant logs:

Review how reactions are stored in message objects and how username changes or account deletion are handled. Reproduce both cases, then verify that reactions are transferred or removed as expected; the issue provides no file or test names.