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)

build/docker/nginx/default.conf

@@ -1,6 +1,7 @@
 upstream app_cluster {
-  server leader:8065 fail_timeout=5s max_fails=10;
-  server follower:8065 fail_timeout=5s max_fails=10;
+  server leader:8065 fail_timeout=10s max_fails=10;
+  server follower:8065 fail_timeout=10s max_fails=10;
+  server follower2:8065 fail_timeout=10s max_fails=10;
 }
 
 server {
@@ -9,6 +10,7 @@ server {
   location ~ /api/v[0-9]+/(users/)?websocket$ {
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection "upgrade";
+    proxy_http_version 1.1;
     client_max_body_size 50M;
     proxy_set_header Host $http_host;
     proxy_set_header X-Real-IP $remote_addr;
@@ -25,6 +27,7 @@ server {
     client_max_body_size 50M;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection "upgrade";
+	proxy_http_version 1.1;
     proxy_set_header Host $http_host;
     proxy_set_header X-Real-IP $remote_addr;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;