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/utils.go

@@ -195,6 +195,11 @@ func GetMillisForTime(thisTime time.Time) int64 {
 	return thisTime.UnixNano() / int64(time.Millisecond)
 }
 
+// GetTimeForMillis is a convenience method to get time.Time for milliseconds since epoch.
+func GetTimeForMillis(millis int64) time.Time {
+	return time.Unix(0, millis*int64(time.Millisecond))
+}
+
 // PadDateStringZeros is a convenience method to pad 2 digit date parts with zeros to meet ISO 8601 format
 func PadDateStringZeros(dateString string) string {
 	parts := strings.Split(dateString, "-")