BR tags in mermaid not rendering

[Example of the problem](https://hackmd.io/s/Bk4fENHVX#)

# Example code

```
sequenceDiagram
participant Joe
participant Alice
Joe->>Alice: Foo
Note over Alice: Bar
Baz
Note over Joe,Alice: That note above should have had a carriage return between Bar and Baz
```

`
` tags are not rendered in [Mermaid Notes](https://mermaidjs.github.io/sequenceDiagram.html#Notes). This may be because of the version of mermaid (7.1.2) that is being used on HackMD.

Over at https://mermaidjs.github.io/mermaid-live-editor/ it renders correctly. [Here's an example](https://mermaidjs.github.io/mermaid-live-editor/#/view/eyJjb2RlIjoic2VxdWVuY2VEaWFncmFtXG4gICAgcGFydGljaXBhbnQgSm9lXG4gICAgcGFydGljaXBhbnQgQWxpY2VcbiAgICBKb2UtPj5BbGljZTogRm9vXG4gICAgTm90ZSBvdmVyIEFsaWNlOiBCYXI8YnIvPkJhelxuICAgIE5vdGUgb3ZlciBKb2UsQWxpY2U6IFRoYXQgbm90ZSBhYm92ZSBzaG91bGQgaGF2ZSBoYWQgYSBjYXJyaWFnZSByZXR1cm4gYmV0d2VlbiBCYXIgYW5kIEJheiIsIm1lcm1haWQiOnsidGhlbWUiOiJkZWZhdWx0In19).

Note : I was confused by a bug in mermaidjs (knsv/mermaid#702) which allows `
` work but `
` (with a space) fails. My original bug had more text because of this confusion.