Fix documentation related to `config.assets.cache_store` [ci skip]

- sprockets-rails no longer supports customizing cache store after
  rails/sprockets-rails@ecaeb27 using `config.assets.cache_store`.
- Instead we need to configure it using block syntax.
- Fixes #19835.

Added references option to join tables

Avoid extra `show variables` in migration

Fixes issue #22960
When creating join tables with the command

    rails g migration CreateJoinTableShowroomUser showroom:references user:references

The migration will use references to create the joins and output:

     class CreateJoinTableShowroomUser < ActiveRecord::Migration
       def change
         create_join_table :showrooms, :users do |t|
           t.references :showroom, index: true, foreign_key: true
           t.references :user, index: true, foreign_key: true
         end
       end
     end

This allows for proper refrences with indexes and foreign keys to be easily used when
adding join tables. Without `:refrences` the normal output is generated:

    class CreateJoinTableShowroomUser < ActiveRecord::Migration[5.0]
      def change
        create_join_table :showrooms, :users do |t|
          # t.index [:showroom_id, :user_id]
          # t.index [:user_id, :showroom_id]
        end
      end
    end

`initialize_schema_migrations_table` is called in every migrations.

https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/migration.rb#L1080
https://github.com/rails/rails/blob/v5.0.0.beta1/activerecord/lib/active_record/schema.rb#L51

This means that extra `show variables` is called regardless of the
existence of `schema_migrations` table.

This change is to avoid extra `show variables` if `schema_migrations`
table exists.

Make to primary key instead of an unique index for internal tables

None of the other components use them, so we should be consistent.

Remove duplicated composite primary key tests

Extract `ExplainPrettyPrinter` to appropriate files

This reverts commit 96355e87, reversing
changes made to a00c36fe.

See https://github.com/rails/rails/pull/23366#issuecomment-177714429

Use `t.index` in `create_table` instead of `add_index` in test schema

Added test for backward compatibility of null constraints on timestamp columns

`OID::Money.precision` is unused since #15239

Update Gemfile.lock

- Leftover from https://github.com/rails/rails/commit/93abf58787396661230f31c7a2f58c18f30dbec9.

Remove `case macro` from `calculate_constructable`

Rails has abstract Reflection classes (`MacroReflection`,
`AssociationReflection` etc.) and concrete Reflection classes
(e.g. `HasManyReflection`, `HasOneReflection` etc.).
In many case `calculate_constructable` returns `true`, so
change `calculate_constructable` to always return `true` and
override this method if necessary.

clarify the touch true option does not trigger after_save/update [ci skip]

Move Action Cable back to the main build

I've gotten tripped up more than a few times on this, thinking that
using `belongs_to` with `touch: true` would trigger my after_save or
after_update callbacks. The same text is in the documentation for the
touch method itself, but I think its helpful to repeat it again here.
It might save people some time.

Update middleware docs regarding ActionDispatch::LoadInterlock [ci skip]

make rake proxy work in rails engines

Each concrete classes have responsibility to return `association_class`

Action Cable Assets Compilation redux

remove backward compatibility code for Minitest 4

Remove odd ` [ci skip]