Services can be configured in `config/storage.yml` with a new key
`public: true | false` to indicate whether a service holds public
blobs or private blobs. Public services will always return a
permanent URL.

Deprecates `Blob#service_url` in favor of `Blob#url`.

Previous behavior would only set host, which didn't work correctly
if the default_url_options contained the protocol or the port.

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.

* Force content-type to binary on service urls for relevant content types

We have a list of content types that must be forcibly served as binary,
but in practice this only means to serve them as attachment always. We
should also set the Content-Type to the configured binary type.

As a bonus: add text/cache-manifest to the list of content types to be
served as binary by default.

* Store content-disposition and content-type in GCS

Forcing these in the service_url when serving the file works fine for S3
and Azure, since these services include params in the signature.
However, GCS specifically excludes response-content-disposition and
response-content-type from the signature, which means an attacker can
modify these and have files that should be served as text/plain attachments
served as inline HTML for example. This makes our attempt to force
specific files to be served as binary and as attachment can be easily
bypassed.

The only way this can be forced in GCS is by storing
content-disposition and content-type in the object metadata.

* Update GCS object metadata after identifying blob

In some cases we create the blob and upload the data before identifying
the content-type, which means we can't store that in GCS right when
uploading. In these, after creating the attachment, we enqueue a job to
identify the blob, and set the content-type.

In other cases, files are uploaded to the storage service via direct
upload link. We create the blob before the direct upload, which happens
independently from the blob creation itself. We then mark the blob as
identified, but we have already the content-type we need without having
put it in the service.

In these two cases, then, we need to update the metadata in the GCS
service.

* Include content-type and disposition in the verified key for disk service

This prevents an attacker from modifying these params in the service
signed URL, which is particularly important when we want to force them
to have specific values for security reasons.

* Allow only a list of specific content types to be served inline

This is different from the content types that must be served as binary
in the sense that any content type not in this list will be always
served as attachment but with its original content type. Only types in
this list are allowed to be served either inline or as attachment.

Apart from forcing this in the service URL, for GCS we need to store the
disposition in the metadata.

Fix CVE-2018-16477.

`ActiveStorage::Blob#download` and `ActiveStorage::Blob#open` raise
`ActiveStorage::FileNotFoundError` when the corresponding file is missing
from the storage service. Services translate service-specific missing
object exceptions (e.g. `Google::Cloud::NotFoundError` for the GCS service
and `Errno::ENOENT` for the disk service) into
`ActiveStorage::FileNotFoundError`.

Closes #32193.

Match other services, which all use a 5 MB chunk size.

Trying to pass the current request down to the service so that it can
create full urls instead of paths makes the API messy so use a model
based on ActiveSupport::CurrentAttributes to provide the current host
to services that need it (primarily the disk service).

Fixes #32129.

`content_type` parameter is before `disposition` parameter.

And enable `context_dependent` of Style/BracesAroundHashParameters cop.

* Use `+` instead of backquote.
* Remove escape from class to be link

The reasons for this commit are:

- uniformity with the other Rails libraries
- (possibly) behave better with respect to autoloading
- fix the index in the generated documentation

Before this commit, run `rake rdoc` generates this left sidebar
(ActiveStorage entries are indexed twice, both inside and outside the module):

<img width="308" alt="before" src="https://user-images.githubusercontent.com/10076/28939523-7c087dec-7846-11e7-9289-38ed4a2930cd.png">

After this commit, run `rake rdoc` generates this left sidebar:
(ActiveStorage entries are only indexed inside the module):

<img width="303" alt="after" src="https://user-images.githubusercontent.com/10076/28939524-7c090be0-7846-11e7-8ee5-29dfecae548e.png">

We are already removing the braces around hash parameters in the last
argument in other places so we should not change the entire codebase
because of two places.