PLT-4938 Add app package and move logic over from api package (#4931)

* Add app package and move logic over from api package

* Change app package functions to return errors

* Move non-api tests into app package

* Fix merge

app/command.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"github.com/mattermost/platform/model"
+)
+
+func CreateCommandPost(post *model.Post, teamId string, response *model.CommandResponse) (*model.Post, *model.AppError) {
+	post.Message = parseSlackLinksToMarkdown(response.Text)
+	post.CreateAt = model.GetMillis()
+
+	if response.Attachments != nil {
+		parseSlackAttachment(post, response.Attachments)
+	}
+
+	switch response.ResponseType {
+	case model.COMMAND_RESPONSE_TYPE_IN_CHANNEL:
+		return CreatePost(post, teamId, true)
+	case model.COMMAND_RESPONSE_TYPE_EPHEMERAL:
+		if response.Text == "" {
+			return post, nil
+		}
+
+		post.ParentId = ""
+		SendEphemeralPost(teamId, post.UserId, post)
+	}
+
+	return post, nil
+}