PLT-2525: Render attachment fields similar to Slack

* Render attachment fields similar to Slack

* Add /loadtest json url command

This allows us to easily create test posts with 
more props like Slack attachments

api/command_loadtest_test.go

@@ -221,3 +221,44 @@ func TestLoadTestUrlCommands(t *testing.T) {
 
 	time.Sleep(2 * time.Second)
 }
+
+func TestLoadTestJsonCommands(t *testing.T) {
+	Setup()
+	// enable testing to use /loadtest but don't save it since we don't want to overwrite config.json
+	enableTesting := utils.Cfg.ServiceSettings.EnableTesting
+	defer func() {
+		utils.Cfg.ServiceSettings.EnableTesting = enableTesting
+	}()
+
+	utils.Cfg.ServiceSettings.EnableTesting = true
+
+	team := &model.Team{DisplayName: "Name", Name: "z-z-" + model.NewId() + "a", Email: "test@nowhere.com", Type: model.TEAM_OPEN}
+	team = Client.Must(Client.CreateTeam(team)).Data.(*model.Team)
+
+	user1 := &model.User{TeamId: team.Id, Email: model.NewId() + "corey+test@test.com", Nickname: "Corey Hulen", Password: "pwd"}
+	user1 = Client.Must(Client.CreateUser(user1, "")).Data.(*model.User)
+	store.Must(Srv.Store.User().VerifyEmail(user1.Id))
+
+	Client.LoginByEmail(team.Name, user1.Email, "pwd")
+
+	channel := &model.Channel{DisplayName: "00", Name: "00" + model.NewId() + "a", Type: model.CHANNEL_OPEN, TeamId: team.Id}
+	channel = Client.Must(Client.CreateChannel(channel)).Data.(*model.Channel)
+
+	command := "/loadtest json "
+	if r := Client.Must(Client.Command(channel.Id, command, false)).Data.(*model.CommandResponse); r.Text != "Command must contain a url" {
+		t.Fatal("/loadtest url with no url should've failed")
+	}
+
+	command = "/loadtest json http://missingfiletonwhere/path/asdf/qwerty"
+	if r := Client.Must(Client.Command(channel.Id, command, false)).Data.(*model.CommandResponse); r.Text != "Unable to get file" {
+		t.Log(r.Text)
+		t.Fatal("/loadtest url with invalid url should've failed")
+	}
+
+	command = "/loadtest url https://secure.beldienst.nl/test.json" // Chicken-egg so will replace with mattermost/platform URL soon
+	if r := Client.Must(Client.Command(channel.Id, command, false)).Data.(*model.CommandResponse); r.Text != "Loading data..." {
+		t.Fatal("/loadtest url for README.md should've executed")
+	}
+
+	time.Sleep(2 * time.Second)
+}