This is a simplification over the previous approach. The OP of a
thread is just a normal post like any other in this model, which
allows some code simplifications (and future query convenience).
This took me some time to figure out so it's useful to document in the
commit message.
When chaining messages from actors, the result type of a message (i.e.
the actual `<M as Message>::Result`) is sometimes itself a
`Result<T, E>`.
In many cases this leads to a situation where the return type of a
message sending process is something like (simplified):
Future<Item=Result<Foo, ConverseError>, Error=actix::MailboxError>
Due to the implementation of
`From<actix::MailboxError> for ConverseError` it is possible to use
`.from_err()` on these futures to convert the future's `Error` to
`ConverseError`.
The type `Result` apparently implements `IntoFuture`, which means that
due to some trait magic that's been applied somewhere in the futures
API a call to `flatten()` can "lift" the contained error if the error
types match and give us a "simple"
Future<Item=Foo, Error=ConverseError>
From that point on chaining becomes easy.
This currently breaks error handling in page render flows. To fix it
properly, the database actor should return failable futures instead of
`Result<T>` wrapped in a future.
