Prevent search engine indexing

**Is your feature request related to a problem? Please describe.**
Paul instances should not be indexed by search engines. We have reports that at least one of them currently is.

**Describe the solution you'd like**
We need to add `noindex,nofollow` robots attribute, either as a tag in [index.html](https://github.com/code4romania/paul/blob/main/client/public/index.html) or standalone in a robots.txt file.