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)

shared/mlog/default.go

@@ -33,6 +33,10 @@ func defaultLog(level, msg string, fields ...Field) {
 	}
 }
 
+func defaultIsLevelEnabled(level LogLevel) bool {
+	return true
+}
+
 func defaultDebugLog(msg string, fields ...Field) {
 	defaultLog("debug", msg, fields...)
 }