Resolve "[Rails5] Found new routes that could cause conflicts with existing namespaced routes"

Closes #48010

See merge request gitlab-org/gitlab-ce!20015

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

Update the monkey-patch to make it work in Rails 5.

Fix table_exists? to be Rails5 compatible

Closes #45107

See merge request gitlab-org/gitlab-ce!19418

Use data_source_exists? where possible instead of table_exists? in order to be Rails5 compatible

It fixes Rails 5.0 deprecation flooding like:

```
DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
them to actual class references.  For example:

  "::Gitlab::Middleware::ReadOnly" => Gitlab::Middleware::ReadOnly

 (called from <top (required)> at /builds/gitlab-org/gitlab-ce/config/environment.rb:11)
DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
them to actual class references.  For example:

  "ActionDispatch::Static" => ActionDispatch::Static

 (called from <top (required)> at /builds/gitlab-org/gitlab-ce/config/environment.rb:11)
DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
them to actual class references.  For example:

  "Gitlab::Testing::RequestBlockerMiddleware" => Gitlab::Testing::RequestBlockerMiddleware

 (called from <top (required)> at /builds/gitlab-org/gitlab-ce/config/environment.rb:11)
DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
them to actual class references.  For example:

  "ActionDispatch::Static" => ActionDispatch::Static

 (called from <top (required)> at /builds/gitlab-org/gitlab-ce/config/environment.rb:11)
DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
them to actual class references.  For example:

  "Gitlab::Testing::RequestInspectorMiddleware" => Gitlab::Testing::RequestInspectorMiddleware

 (called from <top (required)> at /builds/gitlab-org/gitlab-ce/config/environment.rb:11)
```

Clean up stale Prometheus metrics in before_exec unicorn hook

See merge request gitlab-org/gitlab-ce!19683