Escape `_` during serialization of wysiwyg content

[d3m1d0v]

Description

Wysiwyg serializer does not escape _ symbol in content. This may cause differences in the display in wysiwyg and in the html rendered in makrdown-it. For example in case "_em_": when writing in wysiwyg, this content will have underscores, but after changing the mode to markup and back, in wisywig it would be "em". Markdown-it parse it as italic emphasis.

Proposal

Need to add the underscore character _ to the list for escaping in serializer to markdown