fix: Do only convert linebreaks to br tags in p blocks

[krille-chan]

This changes the behavior
of the markdown method to
only convert linebreaks inside
of p blocks. I found no better
solution yet for the problem
as otherwise also lists
will have linebreaks between
the list items. Unfortunately
the default linebreak syntax
seems not to fulfill our needs.

[nico-famedly]

Ah, this breaks some tests. I guess this needs fixes :)