Support full syntax in thread messages

Support full syntax that is supported in main messages.

In particular, these features are missing (marked with ❌):

syntax	example	currently supported
emoji	:cry:	❌
ordered list	1. list	✅
unordered list	- list	✅
italics, bold	*text*, **text**	✅
ATX/Setext heading	# heading...	✅
blockquote	> blockquote	✅
horizontal rule	---	✅
link	[Link](http://a.com)	✅
image		✅
inline code	`code`	✅
block code	```javascript	✅*

*block code is currently supported only via copy/paste because multiline composing (shift + enter) in threaded messages doesn't work at the moment.