[Rails5] Fix ActionCable '/cable' mountpoint conflict

Since Rails 5.0 the new framework has been added. It's called ActionCable.
It brings WebSockets support to rails applications.
By default the mountpoint of WebSocket requests is `/cable` [1].

GitLab allows using top level names as namespaces or usernames.
For example, `gitlab.com/cable` at this moment leads to a user with the
nickname `cable`.

This commit changes ActionCable's mountpoint to a reserved top level
word `-`.
This is just a stub and should not be be used in real work.

Please set correct mountpoints for each environments when configuring
ActionCable for real using.

[1]: https://github.com/rails/rails/blob/5-0-stable/actioncable/lib/action_cable.rb#L38

changelogs/unreleased/blackst0ne-rails5-found-new-routes-that-could-cause-conflicts-with-existing-namespaced-routes.yml

+---
+title: "[Rails5] Fix ActionCable '/cable' mountpoint conflict"
+merge_request: 20015
+author: "@blackst0ne"
+type: fixed

config/application.rb

@@ -57,6 +57,13 @@ module Gitlab
     # Configure the default encoding used in templates for Ruby 1.9.
     config.encoding = "utf-8"
 
+    # ActionCable mount point.
+    # The default Rails' mount point is `/cable` which may conflict with existing
+    # namespaces/users.
+    # https://github.com/rails/rails/blob/5-0-stable/actioncable/lib/action_cable.rb#L38
+    # Please change this value when configuring ActionCable for real usage.
+    config.action_cable.mount_path = "-" if rails5?
+
     # Configure sensitive parameters which will be filtered from the log file.
     #
     # Parameters filtered: