Add robots.txt to disallow all paths

[alangrainger]

I wanted to do this to prevent constant 404's from crawlers.

For my own hosted version, I absolutely want to deny all paths to robots. I'm not certain whether you also want that behaviour, or if there are some paths you wish to be crawled.

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If the PR isn't acceptable, there is a workaround to add a robots.txt through your compose file:

    volumes:
       - ./robots.txt:/kutt/static/robots.txt:ro