Fixed fastforwarding closing videos' fullscreen mode
Fixed fastforwarding closing videos' fullscreen mode

This commit stops the MessageView from adding the attachment more than once. Previously an attachment was appended to the MessageView every time an update
event was emitted, which happens when forwarding.

// FREEBIE

nrizzio/Signal-Desktop,nrizzio/Signal-Desktop,nrizzio/Signal-Desktop,nrizzio/Signal-Desktop