Fix slack's icon tray missing in Ubuntu 17.04

After upgrading my computer to Ubuntu 17.04, the Slack's icon try gone missing. Here is a quick fix:

1. Edit /usr/share/applications/slack.desktop, and change the following line:

Exec=/usr/binslack --disable-gpu %U

to

Exec=env XDG_CURRENT_DESKTOP=Unity /usr/bin/slack --disable-gpu %U

2. Restart slack to see the icon tray appears