Fix markdown editor bugs

[avele]

• clear block button doesn't work
• 4 spaces before text make it "code-text" but after save its regular text
• quote doesn't work

[neongreen]

Here's the current 4-space code behavior. It's better but still somewhat broken.

How it looks in the editor

How it looks when rendered

Data on the server

One-line sample:

    Hello world

Two-line code sample:

    This is line one
<tab><tab>And this is line two

Ok.

[avele]

Fixed in @ #296