Allow specifying users who are read-only.

Define them under "ADDITIONAL_READ_ONLY_USERS" in config, they will be
updated to the latest table version, and searched when username=table
name, but won't be added to. Useful for getting more results from users who were in
ADDITIONAL_USERS, but have since deleted their accounts, for example.
Add a conversation API endpoint.
Lookup the initial id, and replies to it.
Avoid duplicate queries, lookup all in_reply_to.