Improve Markdown and text editor

Currently, v 0.21.2, Markdown markup and text editor have some flaws. Most of all it's about linebreaks and easily formatting.

Linebreaks

• A simple linebreak is not displayed as so (inherent to Markdown) and this is really disgusting because user needs to create a double linebreak. It would be great to modify this; there are several already modified Markdowns.
• In default Markdown, adding to spaces at the end of line creates a simple linebreak. But this is not in Funkwhale.

Easily formatting

Just like many editors, there should be buttons to format characters, but there is not in Funkwhale.

GitLab easily formatting tools:

Write and Preview toggling

When toggling between those views, the downwards extended text editor field configuration is not save:

But should.