ABC-132: sign error page parameters (#8197)

* sign error page parameters

* add comments

app/config_test.go

@@ -6,6 +6,8 @@ package app
 import (
 	"testing"
 
+	"github.com/stretchr/testify/assert"
+
 	"github.com/mattermost/mattermost-server/model"
 )
 
@@ -54,3 +56,10 @@ func TestConfigListener(t *testing.T) {
 		t.Fatal("listener 2 should've been called")
 	}
 }
+
+func TestAsymmetricSigningKey(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+	assert.NotNil(t, th.App.AsymmetricSigningKey())
+	assert.NotEmpty(t, th.App.ClientConfig()["AsymmetricSigningPublicKey"])
+}