Only apply DATABASE_URL for Rails.env

In passing, allow multi-word adapters to be referenced in a URL:
underscored_name must become hyphened-name.

The "DATABASE_URL_*" idea was moving in the wrong direction.

Instead, let's deprecate the situation where we end up using
ENV['DATABASE_URL'] at all: the Right Way is to explicitly include it in
database.yml with ERB.

.. even when the supplied config made no hint that name was relevant.

If the supplied string doesn't contain a colon, it clearly cannot be a
database URL. They must have intended to do a key lookup, so even though
it failed, give the explanatory deprecation warning, and raise the
exception that lists the known configs.

Conveniently, this also simplifies our logical behaviour: if the string
matches a known configuration, or doesn't contain a colon (and is
therefore clearly not a URL), then we output a deprecation warning, and
behave exactly as we would if it were a symbol.

This seems to simplify the operative part. Most importantly, by
pre-loading all the configs supplied in ENV, we ensure the list is
complete: if the developer specifies an unknown config, the exception
includes a list of valid ones.

configuration

As we like ENV vars, also support DATABASE_URL_#{env}, for more obscure
use cases.

The original attempt didn't really fix the problem and wasn't testing the
problematic area. This commit corrected those issues in the original commit.

Also removed the private `enum_mapping_for` method. As `defined_enums` is now a
method, this method doesn't provide much value anymore.

Build the reverse_order on its proper method.

remove check for present? from delete_all method

Fixes #14620

Passing in a blank string is a bug so there's no reason
to check for a blank string.

The reverse_order method was using a flag to control if the order should
be reversed or not. Instead of using this variable just build the reverse order
inside its proper method.

This implementation was leading to an unexpected behavior when using
reverse_order and then applying reorder(nil).

Example:
  Before
    Post.order(:name).reverse_order.reorder(nil)
    # => SELECT "posts".* FROM "posts"   ORDER BY "posts"."id" DESC

  After
    Post.order(:name).reverse_order.reorder(nil)
   # => SELECT "posts".* FROM "posts"

Use connection-specific bytea escaping

In our normal usage, it's rare for this to make a difference... but is
more technically correct.

As well as a spec that proves this is a good idea, let's also add a more
sane-looking one that just covers basic to_sql functionality. There
aren't many places where we actually use escape_bytea, but that's one
that won't be going away.

Ignore order when doing count.

This is necessary because Postgresql doesn't play nice with ORDER BY and
no GROUP BY.

Fixes #14621.

Make enums distinct per class

Remove unused `subclass_controller_with_flash_type_bar` var from flash test.

Added OS X specific commands to installation guide [ci skip]

fix CollectionProxy delete_all documentation

This method no longer returns an array of all records that
have been removed. Correct documentation to reflect this change.

See issue 14546

Recommended using homebrew for installing MySQL and PostgreSQL

Fix setup of adding _flash_types test.