minimal examples: update for CSP best practices

1) update the logos to svg

2) add svg icon for strict security policy where used

3) define new vhost option flag to enforce sending CSP headers
   with the result code

4) add vhost option flag to minimal examples to
   enforce sending CSP where applicable

5) Go through all the affecting examples confirming they
   still work

6) add LWS_RECOMMENDED_MIN_HEADER_SPACE constant (currently
   2048) to clarify when we need a buffer to hold headers...
   with CSP the headers have become potentially a lot
   larger.