Mattermost assigns its own IDs to messages, thus when sending a message
to Mattermost the event_seen key that has to be written must take into
account that ID and not the one that we put in the event (which was the
Matrix event ID)
Note that for XMPP anything can be used as an ID, so using the Matrix
event ID there worked, but it's actually not so good.
With the Postgres backend, strings will use text by default.
Using varchar(255) by default (which is stupid) is done only for sqlite,
which doesn't care about the type anyways.
