MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

store/searchlayer/channel_layer.go

@@ -114,8 +114,8 @@ func (c *SearchChannelStore) RemoveMembers(channelId string, userIds []string) e
 	return nil
 }
 
-func (c *SearchChannelStore) CreateDirectChannel(user *model.User, otherUser *model.User) (*model.Channel, error) {
-	channel, err := c.ChannelStore.CreateDirectChannel(user, otherUser)
+func (c *SearchChannelStore) CreateDirectChannel(user *model.User, otherUser *model.User, channelOptions ...model.ChannelOption) (*model.Channel, error) {
+	channel, err := c.ChannelStore.CreateDirectChannel(user, otherUser, channelOptions...)
 	if err == nil {
 		c.rootStore.indexUserFromID(user.Id)
 		c.rootStore.indexUserFromID(otherUser.Id)

store/searchlayer/file_info_layer.go

@@ -22,7 +22,7 @@ func (s SearchFileInfoStore) indexFile(file *model.FileInfo) {
 				if file.PostId == "" {
 					return
 				}
-				post, postErr := s.rootStore.Post().GetSingle(file.PostId)
+				post, postErr := s.rootStore.Post().GetSingle(file.PostId, false)
 				if postErr != nil {
 					mlog.Error("Couldn't get post for file for SearchEngine indexing.", mlog.String("post_id", file.PostId), mlog.String("search_engine", engineCopy.GetName()), mlog.String("file_info_id", file.Id), mlog.Err(postErr))
 					return