Add examples describing error handling in ActiveSupport::Notification…

Rename header of the gzip paragraph

Mention tiff support in ActiveStorage guides

[ci skip]

Add CHANGELOG.md to actionmailbox gem specification

Bump license years for 2019

Add streams assert methods to ActionCable channel test case

Accommodate case-insensitive filesystems and database collations.

It contains guard/rb-inotify#79, which is what we were waiting for.

Use 6.0 version of `ActiveRecord::Migration` for Action Mailbox

Add Action Mailbox to guides

Since Action Mailbox will be introduced in Rails 6.0,
it makes more sense to generate migration of that version.
Also I changed its test dummy app to use default 6.0 configs.

Now Action Mailbox is responsible for receiving emails.

I added WIP label to that guide since we definitely
want to complement it.

**before**

```
$ ./bin/rails g g
Could not find generator 'g'. Maybe you meant nil?
Run `rails generate --help` for more options.
```

**after**

```
$ ./bin/rails g g
Could not find generator 'g'.
Run `rails generate --help` for more options.
```

Ensure that the app generator skips action mailbox when `--skip-active-record` is given

Related to https://github.com/rails/rails/pull/34816#issuecomment-450378366
Follow up https://github.com/rails/rails/commit/ddaf06779aa51d5d1ca462c21c53f2ed169a0d2f

Remove mention about `receive.action_mailer` from the AS instrumentation guide

Since e3f832a7 `ActionMailer::Base.receive` is
deprecated.

Follow up to d082439c