Clarify documentation by removing charged words [ci skip]

Make ActionView::LookupContext::DetailsKey thread-safe

* Keep consistent verb tenses

[ci skip]

* Add link to "creating your own events"

[ci skip]

* Move section on how to subscribe to events above list of events

The list of events is really long, and most people only care about a
specific event or type of event.

[ci skip]

 Fix route from "(:a)(foo/:b)" when only given :b

Fix route from "(:a)(:b)" when given only :a or :b

Using words such as 'just', 'simply' and 'easy' implies to the reader
that the tasks they are trying to accomplish are tasks that anyone can
do, which might then frustrate them if they find it difficult to
complete.

This commit rephrases some usage of these words to aid understanding in
the readers without making them feel belittled.

[ci skip]

Test extensions to Mail gem

Add changelog entry for https://github.com/rails/rails/pull/37918

Per request from @bogdanvlviv.

Fix typo in ActiveRecord::AttributeMethods::Dirty

Improve contributing guide for RDoc [ci skip]

Remove unused requires

`reporting` was removed at 7e75dc5d.
`singleton_class` was removed at b342d2ba.

Move records and loaded ivar up to load

We should set the `@records` and `@loaded` ivar inside `#load` rather
than in `#exec_queries`. Since `#load` is running `#exec_queries` and
`@records` can only be assigned if `loaded?` is true and `@loaded` can
only be set if `loaded?` is true then `#load` should do the assignment.
This is cleaner but also we came across this while working on an
internal gem and realized that the ivar assignment was happening in the
wrong place.
Co-authored-by: NAaron Patterson <aaron.patterson@gmail.com>

Add ActiveJob::Base.default_retry_jitter config option to specify jitter value

This reverts commit b4ab1f19, reversing
changes made to 9926fd86.

This is causing CI to fail about 50% of the time, and can be reproduced
locally 100% of the time on my machine.

Ex failing build https://buildkite.com/rails/rails/builds/65467#efaa1dd5-aaf4-43a1-a204-d1c42abf614d

Error:

```
SQLite3::BusyException: database is locked (ActiveRecord::StatementInvalid)
```

Introduce class_names helper

Render objects that respond_to render_in in controllers

Fix docs about etag

In https://github.com/rails/rails/pull/36388,
we supported passing objects that `respond_to` `render_in`
to `render`, but _only_ in views.

This change does the same for controllers, as Rails
generally gives the expectation that `render` behaves
the same in both contexts.
Co-authored-by: NAaron Patterson <tenderlove@github.com>

As a follow-up to https://github.com/rails/rails/pull/37872,
this change introduces a class_names view helper
to make it easier to conditionally apply class names
in views.

Before:
<div class="<%= item.for_sale? ? 'active' : '' %>">

After:
<div class="<%= class_names(active: item.for_sale?) %>">

We've been using this helper in the GitHub monolith
since 2016.
Co-authored-by: NAaron Patterson <tenderlove@github.com>

Database URL supports query value with equal sign

Fix input value not properly applied:

- #37872 introduced a regression and you can't do

  ```html.erb
    hidden_field_tag('token', value: [1, 2, 3])
  ```

  This will result in a `<input type="hidden" value=""`>.

  I chose `hidden_field_tag` and the `value` attribute as an example
  but this issue applies to any tag helper and any attributes.

  https://github.com/rails/rails/pull/37872#issuecomment-561806468
  mention that the feature should only apply for "class" attribute.

  This commit fix original intent of #37872

Retain selections with `includes` and `joins`

A querystring value should be allowed to include an equal sign `=`.

This is necessary to support passing `options` for a PostgresSQL connection.

```

development:
  url: postgresql://localhost/railsdevapp_development?options=-cmysetting.debug=on
```

Before this PR, attempting to start the rails process with that configuration would result in an error:

```
> bundle exec rails console
Traceback (most recent call last):
	49: from bin/rails:4:in `<main>'
	48: from bin/rails:4:in `require'
...
	 1: from /rails/activerecord/lib/active_record/database_configurations/connection_url_resolver.rb:58:in `query_hash'
/rails/activerecord/lib/active_record/database_configurations/connection_url_resolver.rb:58:in `[]': invalid number of elements (3 for 1..2) (ArgumentError)
```

After this PR, rails can properly parse the configuration:

```
> bundle exec rails console
Loading development environment (Rails 6.1.0.alpha)
2.6.5 :001 > ActiveRecord::Base.connection.select_all("show mysetting.debug").to_a
   (0.4ms)  show mysetting.debug
 => [{"mysetting.debug"=>"on"}]
```

Added posibility to open a `read_uncommitted` transaction on SQLite:

Don't run AJ after_enqueue / after_perform when chain is halted:

- ### Problem

  ```ruby
    MyJob < ApplicationJob
      before_enqueue { throw(:abort) }
      after_enqueue { # enters here }
    end
  ```
  I find AJ behaviour on after_enqueue and after_perform callbacks
  weird as they get run even when the callback chain is halted.
  It's counter intuitive to run the after_enqueue callbacks even
  though the job wasn't event enqueued.

  ### Solution

  In Rails 6.2, I propose to make the new behaviour the default
  and stop running after callbacks when the chain is halted.
  For application that wants this behaviour now or in 6.1
  they can do so by adding the `config.active_job.skip_after_callbacks_if_terminated = true`
  in their configuration file.

Fix ActiveJob logging when callback chain is halted:

- ### Problem

  ActiveJob will always log "Enqueued MyJob (Job ID) ..." even
  if the job doesn't get enqueued through the adapter.
  Same problem happens when performing a Job, "Performed MyJob (Job ID) ..." will be logged even when job wasn't performed at all.
  This situation can happen either if the callback chain is terminated
  (before_enqueue throwing an `abort`) or if an exception is raised.

  ### Solution

  Check if the callback chain is aborted/exception is raised, and  log accordingly.