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)

model/file_info.go

@@ -61,6 +61,7 @@ type FileInfo struct {
 	HasPreviewImage bool    `json:"has_preview_image,omitempty"`
 	MiniPreview     *[]byte `json:"mini_preview"` // declared as *[]byte to avoid postgres/mysql differences in deserialization
 	Content         string  `json:"-"`
+	RemoteId        *string `json:"remote_id"`
 }
 
 func (fi *FileInfo) ToJson() string {
@@ -105,6 +106,10 @@ func (fi *FileInfo) PreSave() {
 	if fi.UpdateAt < fi.CreateAt {
 		fi.UpdateAt = fi.CreateAt
 	}
+
+	if fi.RemoteId == nil {
+		fi.RemoteId = NewString("")
+	}
 }
 
 func (fi *FileInfo) IsValid() *AppError {