So that the `file_system` option renders correctly on the docs

Closes https://github.com/thoughtbot/paperclip/issues/2253

[ci skip]

* Added support for aws-sdk-s3 gem which is now preferred way to interact with s3.

Reference: https://github.com/aws/aws-sdk-ruby/blob/master/V3_UPGRADING_GUIDE.md#library-maintainer

* Drop support for aws-sdk gem

[ci skip]

There was a bug in the code where if you created a bucket the file would
get read but not rewind on the first pass through (when the bucket
didn't exist). So when we went into the retry logic the file would
already be read and zero bytes would get copied over from the file. This
fixes that by rewinding the file during the retry so it gets correctly
copied over the second time around.

Please see #2527 for details.

* Provide example for s3_host_name

Gives an example, since not everyone knows the format required.

* Shorten line per Hound comment

* Add migration_version variable to migration generator for rails 5 compatibility

* Update syntax to satisfy hound

* Ruby 2.4.0 compatibility (doen't use Fixnum anymore)

* Fix typo

* Use Integer instead of 0.class

[ci skip]

This is a year in the making, with a security patch motivating it.

[ci skip]

Remove the URI adapters. Few people use them by default and they can
allow insight into the internal networks of the server. If you want to
enable them, add (for example) `Paperclip.DataUriAdapter.register` to
your `config/initializers/paperclip.rb` file.

This is related to CVE-2017-0889.

Elsewhere fix CI: it's `s3.us-west-2` now, with a dot.

Exception is the root of Ruby's exception hierarchy, so when you rescue Exception you rescue from everything, including subclasses such as SyntaxError, LoadError, and Interrupt etc which is a bad idea.

Fix: rescue from StandardError.

s3_permissions can no longer be a symbol. Must be a string.
Providing a string causes hard to debug errors in AWS.

Allows `s3_host_alias` to be set to CDNs which support S3 Bucket prefix.

[fixes #2287]

Bug introduced in 1c7d7f66

Per https://github.com/thoughtbot/paperclip/commit/1c7d7f66ceea27cfe604e3a13c2eb87df217d13a#commitcomment-18765681 request

Rebased #2120 to master.
Paperclip duplicates the original files quite a lot as part of its validation process. (#1642, #1326).
When uploading large files (several hundred megabytes to gigabyte range), this becomes a problem: The web server will be busy creating 3 - 4 duplicates on disk, while the app (and potentially the user) are waiting for the upload operation to complete.
This pull request introduces hard links instead of ```FileUtil.cp``` where possible to keep the logic as-is but save time and disk space.

[Amazon S3 Transfer
Acceleration](http://docs.aws.amazon.com/AmazonS3/latest/dev/transfer-acceleration.html)
enables fast, easy, and secure transfers of files over long distances
between your client and an S3 bucket. And
[aws-sdk](https://github.com/aws/aws-sdk-ruby) has allowed us to enable
this feature in version 2.3.0 (more detail is
[here](https://github.com/aws/aws-sdk-ruby/pull/1163)).

You can now pass a configuration option (:use_accelerate_endpoint) when
config Paperclip with S3. You can enable the [Amazon S3 Transfer
Acceleration](http://docs.aws.amazon.com/AmazonS3/latest/dev/transfer-acceleration.html)
by set this option:
```:use_accelerate_endpoint => true```

[fixes #2291]

Adapters now accept an options parameter, that currently specifies
the type of hash digest to use.  The default value remains MD5, but
can be specified to be any OpenSSL-supported digest.  The specs are
modified to reflect that.

The task just reassigns all of the attachments, thereby regenerating
their fingerprints.

* ignore invalid content-disposition header

[fixes #2283]

Fix typo.

Progress on https://github.com/thoughtbot/paperclip/issues/2071.

[closes #2282]

[ci skip]

[closes #2155]

Update NEWS file.

[ci skip]

* Add default `content_type_detector` to `UploadedFileAdapter`

Default `s3_protocol` used to be `http` when `:s3_permissions` are
`:public_read` (default), and `https` when `:s3_permissions` are
anything else.

With an empty String as default, if the page is served over HTTPS,
attachment URL will be HTTPS, and if the page is served over HTTP,
attachment url will be HTTP.

`public-read` is an authorization concept, independent form the
encryption of the HTTP connection used to read the file. As such, the
one shouldn't define the other.

[fixes #2038]