Broken user and channel mentions in blockquotes in chat markdown

[taoeffect]

Problem

Following up on PR #2458, one bug remains (copy/pasting):

It seems the logic for @mentions and channel name mentions is a bit wonky too, as sometimes it works, and sometimes it doesn't:

Markdown:

> @u1 Test #general 

And

> `another () => test` of the
> 
> ```
> emergency () => system
> and
> 
> break
> ```
> 
> But what about
> Paragraph breaks
> That are empty
> 
> Like this one @u1 #general 

() => test

Result:

Github (checking to see if it handles @U1...):

@U1 Test #general

And

another () => test of the

emergency () => system
and

break

But what about
Paragraph breaks
That are empty

Like this one @U1 #general

() => test

Solution

Make it so that user and channel mentions are properly highlighted in this other scenario too.