Implement some post endpoints for APIv4 (#5353)

* Implement POST /posts endpoint for APIv4

* Implement GET /channels/{channel_id}/posts endpoint for APIv4

* Implement GET /posts/{post_id} endpoint for APIv4

* Implement GET /posts/{post_id}/thread endpoint for APIv4

* Skip team get if it's a DM channel in handlePostEvents

api4/apitestlib.go

@@ -26,6 +26,8 @@ type TestHelper struct {
 	TeamAdminUser *model.User
 	BasicTeam     *model.Team
 	BasicChannel  *model.Channel
+	BasicChannel2 *model.Channel
+	BasicPost     *model.Post
 
 	SystemAdminClient *model.Client4
 	SystemAdminUser   *model.User
@@ -97,12 +99,16 @@ func (me *TestHelper) InitBasic() *TestHelper {
 	me.LoginTeamAdmin()
 	me.BasicTeam = me.CreateTeam()
 	me.BasicChannel = me.CreatePublicChannel()
+	me.BasicChannel2 = me.CreatePublicChannel()
+	me.BasicPost = me.CreatePost()
 	me.BasicUser = me.CreateUser()
 	LinkUserToTeam(me.BasicUser, me.BasicTeam)
 	me.BasicUser2 = me.CreateUser()
 	LinkUserToTeam(me.BasicUser2, me.BasicTeam)
 	app.AddUserToChannel(me.BasicUser, me.BasicChannel)
 	app.AddUserToChannel(me.BasicUser2, me.BasicChannel)
+	app.AddUserToChannel(me.BasicUser, me.BasicChannel2)
+	app.AddUserToChannel(me.BasicUser2, me.BasicChannel2)
 	app.UpdateUserRoles(me.BasicUser.Id, model.ROLE_SYSTEM_USER.Id)
 	me.LoginBasic()
 
@@ -188,6 +194,27 @@ func (me *TestHelper) CreateChannelWithClient(client *model.Client4, channelType
 	return rchannel
 }
 
+func (me *TestHelper) CreatePost() *model.Post {
+	return me.CreatePostWithClient(me.Client, me.BasicChannel)
+}
+
+func (me *TestHelper) CreatePostWithClient(client *model.Client4, channel *model.Channel) *model.Post {
+	id := model.NewId()
+
+	post := &model.Post{
+		ChannelId: channel.Id,
+		Message:   "message_" + id,
+	}
+
+	utils.DisableDebugLogForTest()
+	rpost, resp := client.CreatePost(post)
+	if resp.Error != nil {
+		panic(resp.Error)
+	}
+	utils.EnableDebugLogForTest()
+	return rpost
+}
+
 func (me *TestHelper) LoginBasic() {
 	me.LoginBasicWithClient(me.Client)
 }