Slack import doesn't retain message pinning

Description:

When importing from Slack, although all the message history and files are imported, the pinning of messages is lost.

i.e. If a message was pinned in Slack, in Rocket Chat it's not.

Steps to reproduce:

1. Pin a message in a Slack Channel
2. Export the channel for a date range including the pinned message
3. Import the exported .zip file into Rocket Chat

Expected behavior:

The message that was pinned in Slack is still pinned in Rocket Chat

Actual behavior:

No imported messages are pinned.

Server Setup Information:

• Version of Rocket.Chat Server: 3.6.3
• Operating System: RHEL 7
• Deployment Method: tar
• Number of Running Instances: 1
• DB Replicaset Oplog: Enabled
• NodeJS Version: v12.14.0
• MongoDB Version: 4.0.20

Client Setup Information

• Desktop App or Browser Version: Firefox 80.0.1 (64-bit)
• Operating System: Ubuntu

Additional context

Relevant logs:

No files or tests are named in the issue. Locate the Slack import entry point and inspect how exported messages are mapped, particularly their pinning state; done means a pinned Slack message remains pinned after import and the behavior is covered by an appropriate import test.