Improve error message for guides:generate:kindle

With `kindlegen` missing from the PATH, the task
keeps suggesting to `gem install kindlerb`.
This change provides a more meaningful error
message for guides:generate:kindle.

Explicitly load Kernel#y when starting a console

Previously, we relied on the IRB-detection in Psych itself. But that
doesn't work when we're running under spring: the application boots (and
thus psych is required) before we switch to console mode and load IRB.

Fixes #14587.

Revise 'sqlite3:' URL handling for smoother upgrades

Conflicts:
	activerecord/CHANGELOG.md

Avoid a spurious deprecation warning for database URLs

Add ConnectionHelper to refactor tests.

Change order of scope subsections in Active Record Querying guide [ci skip]

In Active Record Querying guide, currently `Merging of scopes` section uses a concept of default scope in its example before it is introduced in the following section titled `Applying a default scope`.

It makes more sense to switch the ordering to introduce default scopes (`Applying a default scope`) first and then go into `Merging of scopes` section where it is used.

Fix default_url_options example in i18n guide

The keys are quite sufficient; we shouldn't be throwing passwords
around.

This is all about the case where we have a `DATABASE_URL`, and we have a
`database.yml` present, but the latter doesn't contain the key we're
looking for.

If the key is a symbol, we'll always connect to `DATABASE_URL`, per the
new behaviour in 283a2ede.

If the key is a string, on the other hand, it should always be a URL:
the ability to specify a name not present in `database.yml` is new in
this version of Rails, and that ability does not stretch to the
deprecated use of a string in place of a symbol.

Uncovered by @guilleiguaran while investigating #14495 -- this actually
may be related to the original report, but we don't have enough info to
confirm.

The option actually needs to be set on action_dispatch. [ci skip]

Change to require all active_support/deprecation since that's the actual
entry point for the deprecation methods.

That which was now relative is now absolute.

Restore the 4.0 behaviour for 'sqlite3:///', but deprecate it. We'll
change to the absolute-path interpretation in 4.2.

The current "correct" spellings for in-memory, relative, and absolute
URLs, respectively, are:

    sqlite3::memory:
    sqlite3:relative/path
    sqlite3:/full/path

Substantially reverses/defers fbb79b51.
Uncovered by @guilleiguaran while investigating #14495, though that
sounds like a different issue.

ActiveSupport::SafeBuffer#prepend inconsistency

Fixed upgrade guide typo

Using `Rails.application.config.cookies_serializer :hybrid` throws an exception, fixed to `=`.

We can conditional define the tests depending on the adapter or connection.

connection.

Expand the query used in #table_exists? to include materialized views in the
kinds of relations it searches.

Also do not use transactional fixtures. We drop the type and the
table after every run, so there is nothing for the transaction to clean up.

Allow postgresql enum_test to be run in random order.

Changed 'so' to 'see'

Creating and dropping similar tables within the same connection causes postgresql to look up old values in the cache of tables which have already been dropped.

* rmc:
  push move_string in to `move`
  combine move_regexp and move_string so we only loop over states once
  do not create memo objects since we'll just throw them away
  only ask if `t` is empty once.

* Clarify what the situation is and what to do.
* Advise loading schema using `rake db:setup` instead of migrating.
* Use a rescue in the initializer rather than extending the error
  message in-place.
* Preserve the original backtrace of other errors by using `raise`
  rather than raising again with `raise error`.

References 0ec45cd1

This is a follow-up fix to f7a6b115 and
06082f66