Show client-side error if wiki page is empty (#17415)

Show client-side error if wiki page is empty (#17415)

* fix: show client-side error if wiki page is empty

Implement a JS, client-side validation workaround for a bug in the upstream
editor library SimpleMDE which breaks HTML5 client-side validation when
a wiki page is submitted.

This allows native, client-side errors to appear if
the text editor contents are empty.

See upstream bugfix report: https://github.com/sparksuite/simplemde-markdown-editor/issues/324

Signed-off-by: David Jimenez <dvejmz@sgfault.com>
Co-authored-by: Lauris BH <lauris@nix.lv>