Revert "Save extended_entities if available."

Needs more thought, as extended_entities only contain media, and not
hashtags etc., :(

This reverts commit c38573994d42a8313d0014e5b69a6f9c69161f34.
Add a conversation API endpoint.
Lookup the initial id, and replies to it.
Avoid duplicate queries, lookup all in_reply_to.
Minor cleanup, add missing backticks in SQL.