Rename AuditEntry#event to AuditEntry#action

I found it confusing that a PaperTrail Version has an `#event` (create, update
or destroy, by default) and an AuditEntry also had an `#event`. I've renamed
`AuditEntry#event` to `AuditEntry#action` to remove this confusion. I think
that `AuditEntry#action` is also more consistent with our use of
`AuditEntry#actor`.
Remove unused path in case statement

PaperTrail's `Version#event` will only be 'create', 'update' or 'destroy' as
we're using the default behaviour.
Remove unused method
Simplify VersionAuditEntry and associated helper

`Version#state` is stored as a verb in the past tense. There's no need to
convert this to a verb in the present tense in `VersionAuditEntry`, only to
convert it back in `Admin::AuditTrailHelper#describe_audit_trail_entry`.
Merge branch 'simplify-audit-trail'

We've been investigating the Edition::AuditTrail as part of our i18n work. The
AuditTrail uses PaperTrail in a way that's incompatible with our desired use
of Globalize3. During the investigation, we realised that there were ways in
which we could simplify the AuditTrail.