Introduce the field 'state.lastreviewed'

We have a field `state.lastchanged`. Which is some times updated automatically (e.g. when a script updates the `state.nodes`. For automating quality checks, and alerting / notifications this field can be used to detect problems with auto-update-scripts (e.g. if it stops to update the file each day ), but this field do not help if we want to detect potential for semantically problems.

For automating quality checks, and alerting / notifications, it would be very helpful to know when a community file was last reviewed for semantical errors by a human - e.g. outdated contact details (old, but syntactially valid e-mail address). With that field we could implement a "Review Reminder Service".

Start by locating the existing `state.lastchanged` field and the community file schema it belongs to. Determine where a human semantic review timestamp would be represented, then verify that the new `state.lastreviewed` field can support the described quality checks and review reminders.

Written by the indexing model from the issue text.