Add extractive QA (aka SQuAD style)

[tholor]

So far we rely on matching questions with the one of existing FAQs.

• Pro: fast + reliable answers
• Con: does only work for the most common questions

=> We could add an extractive QA model + some trustworthy text datasources to handle the long tail of questions. If there's no good match in the FAQs we could forward the request to this extractive QA model

[tholor]

removed the "help wanted" label just because we want help for all the open issues anyway right now :)

[tholor]

We could also think of extractive QA on the CORD-19 dataset. Probably we should then separate in the UI the user groups (general public vs. researchers)