Add Sandstorm activity/notification events (#3743)

.meteor/packages

.meteor/versions

.sandstorm/sandstorm-pkgdef.capnp

.sandstorm/setup.sh

packages/rocketchat-lib/server/functions/sendMessage.coffee

...s/rocketchat-lib/server/lib/sendNotificationsOnMessage.js

packages/rocketchat-lib/server/methods/sendMessage.coffee

packages/rocketchat-sandstorm/package.js

packages/rocketchat-sandstorm/server/events.js

packages/rocketchat-ui/lib/notification.coffee