[Bug] <hr> implemented but not apparant in menu

[schlagmichdoch]

Although markdown has the possiblity to add a horizontal ruler to any page it is not usable directly in humhub.

It is implemented but no <hr> Tag is selectable in the editors menu. It also is not possible to use by typing
___
into a new line directly, as is e.g. with headlines (just type "## ")

Strangely, when ___ is typed into a new line and the document is saved afterwords, everything works as expected.

Is is even shown and serialized correctly when editing the page again:

Create:

Show:

Edit again: