[New] Upvote/Downvote conversation search results

[mrsimpson]

Expected Behaviour

As a user who received a result from the knowledge base's historic conversations, I want to be able to rate the quality of a result.

For a first step, I want to be able to up- and downvote results.

• No limitation for an upper or a lower (even negative) rating shall exist
• The same user shall be able to do this multiple times (though only once an hour, this limitation should only be implemented on the frontend)
• The ranking should be respected when returning results in similar conversations (I'm aware this is the tricky part.)

Remark: The rating does not reflect the quality of the conversation as a whole, but relative to the state of the conversation - in contrast to #268

Actual Behaviour

I don't know how valuable a particular conversation is