[chore] markdown: disable Smartypants rendering (#1111)

* [enhancement] markdown: disable SmartyPantsFractions

fixes #1028

* some fractions are still converted to unicode

to fix that, we might need to disable smartypants indeed in its entirety

* disable smartypants completely

for lack of a better simple solution

internal/text/markdown.go

@@ -71,7 +71,9 @@ func (f *formatter) FromMarkdown(ctx context.Context, markdownText string, menti
 		mentions: mentions,
 		tags:     tags,
 		HTMLRenderer: *blackfriday.NewHTMLRenderer(blackfriday.HTMLRendererParameters{
-			Flags: blackfriday.CommonHTMLFlags,
+			// same as blackfriday.CommonHTMLFlags, but with Smartypants disabled
+			// ref: https://github.com/superseriousbusiness/gotosocial/issues/1028
+			Flags: blackfriday.UseXHTML,
 		}),
 	}