* If it raise error `Azure::Core::Http::HTTPError`, return `nil` instead of `false` in
  `ActiveStorage::Service::AzureStorageService#delete`.

* Other services behave as same as this.

Closes #31073.

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

Add in the content_type option, which is defined as part of the generic
ActiveStorage::Service class.

Without this option, attempts to generate a URL for an attached file
fail with "ArgumentError (unknown keyword: content_type)"

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">