Stumbled upon some difficulties here since I wanted to do stuff correct
without changing the API later.
I chose to separate `post` from `reply` to prevent having a separate
`topic` event. Votes for `reply` are calculated through `m.reaction` but
since it can form a relation with any event we can't enforce votes on
this event type only.
I was not sure if we needed an enum for `post` since we might consider
having different `body` types in the future such as a poll or media
content.

Access token is still absent since I'm still not sure how we will store
them. Another consideration to make is whether to require `room_id` for
replies since we *could* retrieve for the event being replied to and
extract it from there.

…rrors