discord-common.profile: harden & allow notifications

What works:
- Basic functionality
- Receiving notifications
- Voice communication
- Watching streams

What wasn't tested:
- Casting streams
- Opening links
- Tracking/displaying "current activity" as status message
- Apparmor

Notes:
- Discord tries to access system dbus (`[ERROR:bus.cc(399)] Failed to
  connect to the bus: Failed to connect to socket
  /run/firejail/mnt/dbus/system: Permission denied`). I don't know what
  business it has with the system dbus, and didn't notice any problems
  due to that.
- I had one crash after 2h of watching a stream. Probably unrelated.

Fixes #5971.

etc/profile-a-l/discord-common.profile

@@ -7,15 +7,7 @@ include discord-common.local
 #include globals.local
 
 # Disabled until someone reported positive feedback
-ignore include disable-interpreters.inc
-ignore include disable-xdg.inc
-ignore include whitelist-runuser-common.inc
-ignore include whitelist-usr-share-common.inc
 ignore apparmor
-ignore disable-mnt
-ignore private-cache
-ignore dbus-user none
-ignore dbus-system none
 
 ignore noexec ${HOME}
 ignore novideo
@@ -26,6 +18,11 @@ whitelist ${HOME}/.local/share/betterdiscordctl
 private-bin awk,bash,cut,echo,egrep,electron,electron[0-9],electron[0-9][0-9],fish,grep,head,sed,sh,tclsh,tr,which,xdg-mime,xdg-open,zsh
 private-etc @tls-ca
 
+# allow D-Bus notifications
+dbus-user filter
+dbus-user.talk org.freedesktop.Notifications
+ignore dbus-user none
+
 join-or-start discord
 
 # Redirect