Make Rails Facebook-free

If Basecamp is a Facebook-free business, then Rails should be Facebook-free framework.

Ref: https://travis-ci.org/rails/rails/jobs/469956825#L1694

No need to handle if FrozenError is available

Rails 6 requires Ruby 2.5, which introduces `FrozenError`
https://docs.ruby-lang.org/en/2.5.0/NEWS.html

Related to #31520

These were unused since 11af089c and e35b98e6.

Fix app boot for Ruby 2.4

I have a test app that was on Ruby 2.4. When I pulled Rails master the
app no longer would boot because of this change and I saw the following
error:

```
SyntaxError:
/Users/eileencodes/open_source/real_rails/railties/lib/rails/all.rb:18:
syntax error, unexpected keyword_rescue, expecting keyword_end
rescue LoadError
      ^
```

Ruby 2.4 doesn't support removing redundant begins so the real issue is
that this app is on Ruby 2.4 and not on Ruby 2.5. But it's super
confusing for a user to understand the reason the app is failing to boot
is because we need Ruby 2.5.

I added this redundant begin back because we need to give a clearer
error message.

Raise helpful error when role doesn't exist

If you try to call `connected_to` with a role that doesn't have an
established connection you used to get an error that said:

```
>> ActiveRecord::Base.connected_to(role: :i_dont_exist) { Home.first }

ActiveRecord::ConnectionNotEstablished Exception: No connection pool
with 'primary' found.
```

This is confusing because the connection could be established but we
spelled the role wrong.

I've changed this to raise if the `role` used in `connected_to` doesn't
have an associated handler. Users who encounter this should either check
that the role is spelled correctly (writin -> writing), establish a
connection to that role in the model with connects_to, or use the
`database` keyword for the `role`.

I think this will provide a less confusing error message for those
starting out with multiple databases.

Fix Ruby 2.6 `warning: mismatched indentations at 'rescue' with 'def' at 15`.

See https://travis-ci.org/rails/rails/jobs/470890129#L2361

Use BigDecimal provided methods to convert String to BigDecimal

MySQL: `ROW_FORMAT=DYNAMIC` create table option by default

Enable `Style/RedundantBegin` cop to avoid newly adding redundant begin block

Module.delegate rdoc update to clarify :to parameter [ci skip]

Do no watch parent directory of `dirs`

`String#to_d` does not raise an exception if an invalid value is specified.
So can remove exception handling.

```
$ bundle exec ruby -v -rbigdecimal -rbigdecimal/util -e 'p "123,003".to_d'
ruby 2.6.0dev (2018-12-21 trunk 66474) [x86_64-linux]
0.123e3
```

This is a regression test for #34751.

* Fix integer regex deprecation warnings for Ruby 2.6.0
* Define =~ in FakeZone to avoid warnings from Ruby 2.6.0

Follow up #34754

Currently we sometimes find a redundant begin block in code review
(e.g. https://github.com/rails/rails/pull/33604#discussion_r209784205).

I'd like to enable `Style/RedundantBegin` cop to avoid that, since
rescue/else/ensure are allowed inside do/end blocks in Ruby 2.5
(https://bugs.ruby-lang.org/issues/12906), so we'd probably meets with
that situation than before.

https://bugs.ruby-lang.org/issues/14132

- Fix a few deprecation warnings
- Remove testing of `Hash#slice`
- Imporve test of `Hash#slice!`
- Remove mention about `Hash#slice` from the guide

https://bugs.ruby-lang.org/issues/14133

Follow up #34761.

Use native `Array#append`, `Array#prepend`, `Hash#transform_keys`, and `Hash#transform_keys!`

Since Rails 6 requires Ruby 2.5.

https://github.com/ruby/ruby/blob/ruby_2_5/NEWS

Follow up #34754.

Unify _read_attribute definition to use &block

Thanks to ko1, passing block parameter to another method is
significantly optimized in Ruby 2.5.
https://bugs.ruby-lang.org/issues/14045

Thus we no longer need to keep this ugly hack.

Require Ruby 2.5 for Rails 6.

Generally followed the pattern for https://github.com/rails/rails/pull/32034

* Removes needless CI configs for 2.4
* Targets 2.5 in rubocop
* Updates existing CHANGELOG entries for fewer merge conflicts
* Removes Hash#slice extension as that's inlined on Ruby 2.5.
* Removes the need for send on define_method in MethodCallAssertions.

This takes away the following log.

```
Error occurred while parsing request parameters.
Contents:

{:foo=>"heyo"}
```
Pass the correct value as JSON

Since MySQL 5.7.9, the `innodb_default_row_format` option defines the
default row format for InnoDB tables. The default setting is `DYNAMIC`.

The row format is required for indexing on `varchar(255)` with `utf8mb4`
columns.

As long as using MySQL 5.6, CI won't be passed even if MySQL server
setting is properly configured the same as MySQL 5.7
(`innodb_file_per_table = 1`, `innodb_file_format = 'Barracuda'`, and
`innodb_large_prefix = 1`) since InnoDB table is created as the row
format `COMPACT` by default on MySQL 5.6, therefore indexing on string
with `utf8mb4` columns aren't succeeded.

Making `ROW_FORMAT=DYNAMIC` create table option by default for legacy
MySQL version would mitigate the indexing issue on the user side, and it
makes CI would be passed on MySQL 5.6 which is configured properly.

The indexing issue on `utf8mb4` columns is resolved since MySQL 5.7.9.

Single nest config in configuration guides