Add blockquote fence syntax to Markdown

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/16564

Before Markdown rendering happens, this will transform this:

```
Let me quote this here email:

>>>
Dear friend,

How are you?

Greetings,

Me
>>>
```

Into this, saving me from having to prefix all of those lines with `>` manually when I copy some multiline text from another medium:

```
Let me quote this here email:

> Dear friend,
> 
> How are you?
> 
> Greetings,
> 
> Me
```

See merge request !3954