Follow up 34786

```
rails/actionmailbox$ bundle exec rake tests
(snip)
...
rails/activerecord/lib/active_record/persistence.rb:48: warning: in `create!':
the last argument was passed as a single Hash
rails/actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb:21:
warning: although a splat keyword arguments here
...
(snip)
```

Follow up #34786

Import Action Mailbox

Generate doc of methods provided by form builders [ci skip]

Because method arguments are different in the methods provided by form
helpers and form builders, I think these are necessary to prevent
confusion.

Fixes #34787

Host authentication introduced by #33145 allows only "0.0.0.0/0", "::/0"
and "localhost" in the development environment.
https://github.com/rails/rails/blob/4298df00ae6219b9b5b7c40f281d4fa4d66f4383/railties/lib/rails/application/configuration.rb#L33

But `rack-test` use `example.org` for default host.
https://github.com/rack-test/rack-test/blob/6c07bf53adf4a4082e206561f5b9b43142fdc821/lib/rack/test.rb#L13

Therefore, if `hosts` is not specified, host authentication rejects
the request.

In Travis CI, this problem does not occur because `test` is specified
by default in `RAILS_ENV` and `RACK_ENV`.
https://docs.travis-ci.com/user/environment-variables/#default-environment-variables

If user actually use it, env may not always be specified. Explicitly
specify hosts in the file so that it works in any environment.

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