Fixed example of `Rails.application.encrypted` method usage

[ci skip]

We generate master key on `rails new`.
Therefore, if do not add master key to `.gitginore` on `rails new`as
well, there is a possibility that the master key will be committed
accidentally.

Fixes https://github.com/rails/rails/pull/30940

To edit/show encrypted file:

```
bin/rails encrypted:edit config/staging_tokens.yml.enc
bin/rails encrypted:edit config/staging_tokens.yml.enc --key config/staging.key
bin/rails encrypted:show config/staging_tokens.yml.enc
```

Also provides a backing Rails.application.encrypted API for Ruby access:

```ruby
Rails.application.encrypted("config/staging_tokens.yml.enc").read
Rails.application.encrypted("config/staging_tokens.yml.enc").config
Rails.application.encrypted("config/staging_tokens.yml.enc", key: "config/staging.key")
```

Instead of stashing the values in constants.

Allow test_ignored_columns_not_included_in_SELECT column names case insensitive

insensitive

i.e. Oracle database identifier is UPPERCASE unlike other databases.

```ruby
(byebug) query = Developer.all.to_sql
"SELECT \"DEVELOPERS\".\"ID\", \"DEVELOPERS\".\"NAME\", \"DEVELOPERS\".\"SALARY\", \"DEVELOPERS\".\"FIRM_ID\", \"DEVELOPERS\".\"MENTOR_ID\", \"DEVELOPERS\".\"CREATED_AT\", \"DEVELOPERS\".\"UPDATED_AT\", \"DEVELOPERS\".\"CREATED_ON\", \"DEVELOPERS\".\"UPDATED_ON\" FROM \"DEVELOPERS\""
```

Handle `TZInfo::AmbiguousTime` errors

Fix the "International Date Line West" timezone

Make `ActiveSupport::TimeWithZone` match Ruby's handling of ambiguous
times by choosing the later period, e.g.

Ruby:
```
ENV["TZ"] = "Europe/Moscow"
Time.local(2014, 10, 26, 1, 0, 0)   # => 2014-10-26 01:00:00 +0300
```

Before:
```
>> "2014-10-26 01:00:00".in_time_zone("Moscow")
TZInfo::AmbiguousTime: 26/10/2014 01:00 is an ambiguous local time.
```

After:
```
>> "2014-10-26 01:00:00".in_time_zone("Moscow")
=> Sun, 26 Oct 2014 01:00:00 MSK +03:00
```

Fixes #17395.

Update configuring.md

Related to 2e0fe592

It was brought to my attention that the Rails guide suggests using filenames to ensure code load order, so I thought I would suggest a better alternative.

`oldie = 'true'` to `tinyint(1)` column is invalid value for MySQL:

```
Mysql2::Error: Incorrect integer value: 'true' for column 'oldie' at row 1: update horses set oldie = 'true'
```

Update plugin_generator.rb

Fix activesupport/CHANGELOG.md [ci skip]

Fix migration version in doc of #up_only

Performance improvements for add_method_to_attributes!

* Add a #populate method to migrations

* Address rubocop issues

* Rename to #up_only and use #execute in the examples intead of the model

* Update CHANGELOG

[Rich Daley & Rafael Mendonça França]

CI against JRuby 9.1.14.0

JRuby 9.1.14.0 has been released and this version is available on Travis CI.
http://jruby.org/2017/11/08/jruby-9-1-14-0

Update `exists?` documentation [ci skip]

Update layouts_and_rendering.md  [ci skip]

Make it clear that `exists?` can be chained onto a relation

Ensure plugin_generator adds to new line in Gemfile, even if the Gemfile
does not end with an empty line.

[Lisa Ugray, Nikita Penzin]

Disallow raw SQL in dangerous AR methods

Compression has long been available, but opt-in and at a 16kB threshold.
It wasn't enabled by default due to CPU cost. Today it's cheap and
typical cache data is eminently compressible, such as HTML or JSON
fragments.

Compression dramatically reduces Memcached/Redis mem usage, which means
the same cache servers can store more data, which means higher hit
rates.

To disable compression, pass `compress: false` to the initializer.