More demo bugfixes

[ZanSara]

• Partially fix "Enter-to-run". Now works for user queries, but still doesn't for the random questions that are programmatically written in. Streamlit really offers no way to do this, seems impossible even to hack around this limitation.
• Add a proper page title and favicon
• Improved the dataset description a bit
• Fix minor issue with the Haystack version rendering
• Try a fix for a ghostly bug in Retriever occurring in case of many parallel requests hitting the server at the exact same instant.