Follow up of #25702.

[ci skip]fix wrong method name in docs

Fix select tag helper used with Enumerable choices

Make helper methods in tests to private

Both reference id and type should be `NOT NULL` if `null: false` is specified

Set `Capybara.app_host` through `host!`

Adding Foreing Key - Don't worry about building the identifier if name is already present.

Remove a duplicate test of inverse_associations_test in AR

Remove unused `JoinPart#name`

Remove rubygems version lock

Remove unused `left_joins_values` generation

Refactor making join constraints

More friendly exception in nested attributes

This method was only used in the Rails tests and not by other methods in
the Rails simulator. Because it's a no-doc'd class it should be safe to
remove without deprecation.

The only difference between `make_inner_joins` and
`make_left_outer_joins` is the `join_type`.

This was added at #22125 but `left_joins_values` is never used.

This version lock added by 3d890b66.
But original issue is fixed with bundler 1.15.

Ref: https://github.com/rubygems/rubygems/issues/1911#issuecomment-300148516

Update test link in ActionView javascripts README.md.

[ci skip]

[ci skip]

Simplify handling of prerequisites in Getting Started guide [ci skip]

Merge pull request #29072 from dixpac/dixpac/add_documentation_for_validate_and_callback_order_of_execution

Improving docs for callbacks execution order [ci skip]

Telling people about prerequisites, and then telling them a page later
how to check and install those prerequisites, is confusing. This commit
removes the list and just handles the software installation in one
place.

Fixes #28565

When define callbacks latest definition on the same callback/method
overwrites previous ones.

Fix implicit calculations with scalars and durations

Remove obsolete Guides source files [ci skip]

* Nested Model Forms guide is out of date, not linked from index,
  and material is covered in the Form Helpers guide.
* Profiling guide was committed as an outline years ago
  and never actually written.

Fix broken external link in security guide.

see: https://github.com/mperham/sidekiq/blob/master/5.0-Upgrade.md

Fix typo in guides [ci skip]

Make `VALID_DIRECTIONS` to `Set`

Previously calculations where the scalar is first would be converted
to a duration of seconds but this causes issues with dates being
converted to times, e.g:

    Time.zone = "Beijing"           # => Asia/Shanghai
    date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
    2 * 1.day                       # => 172800 seconds
    date + 2 * 1.day                # => Mon, 22 May 2017 00:00:00 CST +08:00

Now the `ActiveSupport::Duration::Scalar` calculation methods will try
to maintain the part structure of the duration where possible, e.g:

    Time.zone = "Beijing"           # => Asia/Shanghai
    date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
    2 * 1.day                       # => 2 days
    date + 2 * 1.day                # => Mon, 22 May 2017

Fixes #29160, #28970.