RTF editor will omit line break when editing

[pullopen]

Way to reproduce:

1. start a new article, using RTF editor.
2. Typing:

   # 123
   **abc**
   

3. save and publish as draft.
4. edit again
5. The content in RTF editor will become:

   # 123 **abc**
   

Meanwhile it won't affect the final result if I don't touch it. It's very confusing.

• Plume version: v0.7.3-dev
• Operating system: Windows 10
• Web Browser: Firefox

[KITAITI Makoto]

Yes... we want to replace it with better one but it takes for a while. We reccomend classic editor for now.

[KITAITI Makoto]

refs: https://git.joinplu.me/Plume/Plume/issues/906