:scissors:
Use alphabetic order
Add application verifier

It is an application global verifier that can be used to generate and
verify signed messages.

See the documentation of ActiveSupport::MessageVerifier for more
information.
Add documentation and CHANGELOG entry to Application#verifier
Add documentation about the Action Dispatch salt configuration

[ci skip]
Wrap 80 columns [ci skip]
Rename verifier to message_verifier
Add missing require
Make possibile to get different message verifiers
No need to configure salts
Don't need to use a controller to test the verifier