Generate permissive or restrictive robots.txt in the config generator

mix pleroma.instance gen will generate robots.txt with 2 options

If user passes the default option y robots.txt will look like

User-Agent: *
Disallow: 

And if they choose restrictive they will get

User-Agent: *
Disallow: /

When the robots.txt is generated if there is one that already exists it will backup the old one

Relates to:

#724 (closed)