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)

.gitignore

@@ -18,11 +18,16 @@ web/static/js/libs*.js
 
 config/active.dat
 config/config.json
+config/logging.json
 /plugins
 
 # Enterprise imports file
 imports/imports.go
 
+#license files
+*.license
+*.mattermost-license
+
 # Build Targets
 .prebuild
 .npminstall