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