Changes to labs to make them more stable or correct.

[MSC-NL]

• Fixed nc no longer running after 1 single connection in SSRF lab.
  

• Consistency in use of spaces instead of tabs.
  

• Changed static IP of CORS to localhost so it will work more consistently.
  

• Added installation instructions for imagemagick on Linux for cmd injection lab.
  

• Slightly refactored PDF generation function in IDOR lab to be a bit more readable (the while implied that 1500 documents would be generated, while only one would be).
  

• Changed IDOR lab to run from 500 instead of 1500. This makes it easier to do when the Community edition of Burp Intruder is used (it's slooooooow).
  

• Small change in error handling (dictionary vs string) in yaml deserialisation lab.
  

• Removed a superfluous form element in CSRF lab.