Zip export feature doesn't support multi-line message

Description:

When you choose "Export Message" -> "Export as File", you can get zipped messages.
But all messages are shown as single-line messages even if it's multi-line messages.
This is a fatal readability issue.

Steps to reproduce:

This is an expected reproduction method with open.rocket.chat, but I faced this in my own server wrote in Server Setup Information.

0. make sure your account has a valid email address
1. sign in to open.rocket.chat
2. make a channel or private group
3. make some message with multiple lines
4. choose "Export Message" from the option in the channel action bar
5. choose "Export as File", a term that contains today, "HTML", and click the "Export" button
6. extract zip from the e-mail
7. check the HTML

Expected behavior:

all messages are shown, including multi-line messages.

Actual behavior:

They're shown, but each multi-line message was shown as a single-line message (via just ignoring new-line characters).
This makes the archive unreadable if there was a user who makes heavy multi-line messages in the channel/room.

Server Setup Information:

• Version of Rocket.Chat Server: 3.12.1
• Operating System: Ubuntu 20.04.2
• Deployment Method: snap
• Number of Running Instances: 1
• DB Replicaset Oplog: enabled
• NodeJS Version: 12.18.4
• MongoDB Version: 3.6.14

Client Setup Information

• Desktop App or Browser Version: Desktop App(Electron) 3.1.1
• Operating System: Windows 10 Pro(arch x64, version 20H2, build 19042.867)

Additional context

Ideally, I think it's better the HTML export feature to support Markdown.

Relevant logs:

none(this problem is about missing-feature, so there is no problematic log)

The issue names no files or tests. Start by tracing the channel action bar's “Export Message” → “Export as File” HTML export flow, then reproduce it with a multi-line message and inspect the extracted HTML. Done means exported messages preserve their line breaks and the existing export behavior remains intact.