fix mysql version check in rename_index

also clarify native rename_index support is >= 5.7, not > 5.7

Make it prominent that after_commit and after_rollback swallow exceptions.

The section that mentions after_commit and after_rollback will swallow exceptions should me made clear. Made that section to be a  instead of a regular paragraph.

implement rename_index natively for MySQL > 5.7

Fix an activesupport inflector test changing the inflector but not reverting all its changes afterwards

/cc @fxn

Extract **notable changes**, **deprecations** and **removals** from
each CHANGELOG.

I tried to reference the commits and pull requests for new features
and deprecations.

In the process I also made some minor changes to the CHANGELOGS.

The 4_1_release_notes guide is declared WIP.

because it is only used in
`activerecord/test/cases/adapters/oracle/synonym_test.rb`

See rails/rails#13054

Remove leftover Oracle tests.

fixed typo `destory` => `destroy` [ci skip]

Deprecate ActiveSupport::TestCase.describe

Provide message to define ActiveSupport::TestCase.name instead.
Allow calling describe with a block, which Minitest::Spec does.

[ci skip] removed singulars section from classify doc

Conflicts:
	activesupport/CHANGELOG.md

unnecessary checking of `size` with `second regex` if matched with former regex

Link for "New Post" is added in Getting Started guide [ci skip]

Dropped yajl from Gemfile

Requires JSON gem version 1.7.7 or above as it contains an important security fix.

[ci skip]removed obsolete information about `options` parameter in create method

[ci skip] used new syntax for scopes

The user is expected to explicitly convert the value into an
AS::Duration, i.e. `5.ago` => `5.seconds.ago`

This will help to catch subtle bugs like:

  def recent?(days = 3)
    self.created_at >= days.ago
  end

The above code would check if the model is created within the last 3
**seconds**.

In the future, `Numeric#{ago,until,since,from_now}` should be removed
completely, or throw some sort of errors to indicate there are no
implicit conversion from `Numeric` to `AS::Duration`.

Also fixed & refactor the test cases for Numeric#{ago,since} and
AS::Duration#{ago,since}. The original test case had the assertion
flipped and the purpose of the test wasn't very clear.

security fix.

- Earlier(line number 862) it is said to add <%= link_to 'New post', new_post_path %> on posts index page before table tag. But when posts index page html is shown, "New Post" link is missing.
So aaded "New Post" link in this commit

Fix bug when validating the uniqueness of an aliased attribute.
Conflicts:
	activerecord/CHANGELOG.md

JSON encoder refactor