Merge branch 'mattermost_websocket_proxying' into 'master'

Add websocket proxying to nginx config of mattermost.

Added websocket proxying to Nginx mattermost config according to the [official nginx doc](http://nginx.org/en/docs/http/websocket.html).

Fixes #740

See merge request !456

files/gitlab-cookbooks/gitlab/templates/default/nginx-gitlab-mattermost-http.conf.erb

@@ -8,6 +8,11 @@ upstream gitlab_mattermost {
   server 127.0.0.1:<%= @service_port %>;
 }
+map $http_upgrade $connection_upgrade {
+  default upgrade;
+  ''      close;
+}
+
 <% if @https && @redirect_http_to_https %>
 server {
 <% @listen_addresses.each do |listen_address| %>
@@ -63,6 +68,10 @@ server {
     <% if @https %>
     proxy_set_header   X-Forwarded-Ssl     on;
     <% end %>
+
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection $connection_upgrade;
+
     proxy_pass http://gitlab_mattermost;
   }