Fixes #22675

Allow channel identifiers and also data with no backslahes/escaping to be accepted by
the subscription storer.

This change makes ping into a message type, which
makes the whole protocol a lot more consistent.

Also fixes hacks on the client side to make this all
work.

In order to eliminate the dependecy of CoffeeScript.

* Fix typos/grammar errors
* Make capitalization/naming consistent

If we're deferring one, we should defer the other too.

Fixes #23471

This also marks Action Cable routes as internal to Rails.

Similar to the channel streaming side, these values must be strings for
ActionCable to behave as expected. The conversion will allow users to
send string-convertible values and get the expected behavior.

ActionCable does some things behind the scenes that expects these
"broadcasting"s or "channel"s to be strings. However it's not
immediately obvious that the value must be a string. So adding this
conversion ensures things work as expected.

[ci skip]

Adds changelog headers for beta3 release

Ensures we don't get "onclose" events from a previous WebSocket that was in the "closing" state

We are seeing cases where the websockets get stuck in the 'closing' state
after a tab has been in background for a while. So lets treat those websockets
as closed.

Once RubyGems 2.5.0 is required, then the duplicated files can be
removed, and symlinks can be used instead.

[ci skip]

Before this commit, the `unsubscribed` callbacks in Action Cable server
side channels were never called. This is because when a WebSocket
"goodbye" message was sent from the client, the Action Cable server
didn't properly clean up after the now closed WebSocket. This means that
memory could possibly skyrocket with this behavior, since part of this
commit is to properly remove closed subscriptions from the global
subscriptions hash. Say you have 10,000 users currently connected, and
then all 10,000 disconnect -- before this patch, Action Cable would
still hold onto information (and Ruby objects!) for all of these now
dead connections.

The Event hack was too much of a hack: on actually thinking about it,
there's a rather obvious race.

When running the ActionCable server in development I get a lot of output
in my logs, this commit sets a maximum length of 300 characters for a
broadcast log message.

This PR checks all active Action Cable documentation for typos and other
fixes. It aims to make sure that when Rails 5 is released, that the
Action Cable docs are up to snuff with the other documentation included
with Rails.

[ci skip]

The `WorkerTest`'s `Receiver` is imporsonating an `ActionCable::Connection::Base`, but
just delegates the logger to `ActionCable.logger`.

This creates a mismatch as the connection requires the logger to be a
`TaggedLoggerProxy`'ied logger, while the server doesn't.

Thus to ensure an exception isn't raised when the worker tries to call `tag`
other tests have to assign a proxied logger to their test server.

Instead of forcing change on other tests, have Receiver adhere to the connection
contract and use a `TaggedLoggerProxy`.

As a consequence remove more setup from the tests.

Instead of depending on ApplicationCable::Connection being defined at initialize
we should inject it in the Railtie.

Thus we can kill more setup in the tests too.

We were explicitly referencing Rails.root in ActionCable::Server::Configuration.initialize,
thereby coupling ourselves to Rails.

Instead add `app/channels` to Rails' app paths and assign the existent files
to `channel_paths`.

Users can still append to those load paths with `<<` and `push` in `config/application.rb`.

This means we can remove the custom `Dir` lookup in `channel_paths` and the Rails
and root definitions in the tests.