Blacklist System as Username (#9060)

adapted tests

model/user.go

@@ -565,6 +565,7 @@ var restrictedUsernames = []string{
 	"all",
 	"channel",
 	"matterbot",
+	"system",
 }
 
 func IsValidUsername(s string) bool {

model/user_test.go

@@ -272,6 +272,7 @@ var usernames = []struct {
 	{"spin'punch", false},
 	{"spin*punch", false},
 	{"all", false},
+	{"system", false},
 }
 
 func TestValidUsername(t *testing.T) {