ActionCable: Allow adding custom paths for channels

Bump mail gem constraint from [~> 2.5, >= 2.5.4] to ~> 2.6

Fix ActiveRecord::Relation#cache_key for relations with no results

Be consistent in testing outputs from railties test and use /bin/rails

Eliminate the EventMachine dependency

Update the exception of format constraint in routes

This fixes ⚠ previous definition of download was here

Be consistent in testing outputs from railties test and use /bin/rails everywhere(the default behaviour now) instead of mix of /bin/rake /bin/rails everywhere
[Ryo Hashimoto & Vipul A M]

Mails downstream dependency (Mime-types) has been shown to decrease
memory usage significantly in its 3.0 release. This memory decrease
will be a big win for users upgrading to Rails 5.

Lets nudge users to upgrade Mail alongside Rails.

Pare back default `index` option for the migration generator

- Using `references` or `belongs_to` in migrations will always add index
  for the referenced column by default, without adding `index:true` option
  to generated migration file.
- Users can opt out of this by passing `index: false`.
- Legacy migrations won't be affected by this change. They will continue
  to run as they were before.
- Fixes #18146

We're no longer doing our work in the EM event loop, so errors are quite
unlikely, and if they do occur, they're not really our responsibility to
handle.

(as adapted to use concurrent-ruby / nio4r instead of eventmachine)

ActionCable Adapter tests

Rails::Generators::Actions#gem should work even if frozen string is passed as argument

- Fixes #23137.

Just like their ActiveJob equivalents, these only work within the
current process.

Per https://github.com/rails/rails/issues/20264
[ci skip]

Because of the changes in #22967 the assumption in #18907 is no longer
true because the internal metadata feature for Active Record requires
a working environment.

Define `:foreign_type` as a valid option in `SingularAssociation`

This fixes incorrect assumptions made by e991c7b8 that we can assume the
DB is already casting the value for us. The enum type needs additional
information to perform casting, and needs a subtype.

I've opted not to call `super` in `cast`, as we have a known set of
types which we accept there, and the subtype likely doesn't accept them
(symbol -> integer doesn't make sense)

Close #23190

The Time.use_zone method will only affect ActiveSupport::TimeWithZone
instances created inside of the block passed to Time.use_zone. This
could be confusing when fetching a model and then reading the attribute
before the block and it not changing afterwards because Active Record
caches the conversion from the database value.

Since changing the behavior of Active Record to recreate the value on
every attribute read is an expensive operation the best we can do is
to document the issue.

Fixes #23195.

[ci skip]

prevent --skip-action-cable from erroring when --api option is used

fix path of Action Cable configuration file [ci skip]

This changed in #22950.

Move ApplicationCable::Channel section of readme

I believe the paragraph starting with "This relies" belongs with the one above it.

Rails 5.0+ rake commands implemented in Rails Framework itself. We pr…

Use the correct reference to the redis connection

Fixes #23193

This was throwing `There was an exception - NameError(undefined local variable or method `hi_redis_conn' for #<ActionCable::SubscriptionAdapter::Redis:0x007fb1449e2b70>)` on unsubscribe.