swarm: add reaction support

This is a pretty trivial patch as all the necessary logics are
already supported by the deamon.
A client can use sendMessage with flag = 2 to add a reaction.
This only adds a "react-to" in the body of the message that the
client can interpret.
To remove a reaction, the client can use editMessage (and set the
body to "" for the id of the reaction), also there is no limit
on the content of the reaction and multiple reactions can be added
to the same message.
For non compatible clients, it will be shown as a simple text message
as it's the same type.

Change-Id: I7b13d32771109118b94ed17d0b918e66487e94bb