Improve markdown span custom tag

[RFSH]

The current span tag implementation is based on "subscript". In case of subscript, the text between the special tags must be inline and must be treated as normal text (it shouldn't apply markdown). But that's not necessarily true for span and we should be able to use span in combination with other markdown tags.

The current implementation,

• Doesn't allow usage of span inside another span.
• Basically "escapes" the content of span and shows the content as is (without rendering markdown). So for example we cannot have a link inside the span.

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This came up as part of #919 where we want a tag that does the markdown escaping for us.