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
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
Update the releases notes
Test if two different verifiers don't share the same secret
Fix typos
Make salt argument required for message verifier
Hightlight message verifier in the release notes [ci skip]
Merge pull request #12995 from rails/application-verifier

Add Application#message_verifier method to return a message verifier