Issue: Extend Forum Marking

We have already extended forum marking to show that new articles were created. Since edits are so important to a wiki, it would be useful to have another forum icon to show that new edits have been made.

Added a green version of the icon. Piggybacked a new field $forum['vault_lastedit'] onto the $lastarticlearray function, so a very minimal amount of new code was added. The new article icon takes precedence over this icon, which takes precedence over the new comment icon. This is not handled by the forum data manager. It is directly updated when saving an edit. Edits marked as "minor" will not update this value.