Replace EpicEditor with SimpleMDE

[fenichelar]

SimpleMDE Markdown Editor has a toolbar that is much more user friendly. This also resolves the issue caused by EpicEditor not loading if the data was undefined.

[xmiao]

When moving a block up or down with the arrow button, the text in the editor will be gone.

[xmiao]

Adding one more else branch should be sufficient.

    // If using SCEditor, update the WYSIWYG
    if(this.sceditor_instance) {
      this.sceditor_instance.val(sanitized);
    }
    else if(this.SimpleMDE) {
        this.SimpleMDE.value(sanitized);
    }
    else if(this.ace_editor) {
      this.ace_editor.setValue(sanitized);
    }

[fenichelar]

@xmiao Fixed as suggested.