Track last update

[jesseleite]

• Add new updateLastModified() functionality to entries and terms.
• Add new touch() method similar to Eloquent's.
• Add new track_last_update config option to disable updated_at and updated_by tracking. Closes Ability to disable "updated_by" on save #1339.

[jasonvarga]

Can we move the config option into users.phpsystem.php?

This is feature isn't really cp specific.

You can call updateLastModified() from anywhere, even outside the CP (for instance, in Workshop down the road)

[jasonvarga]

Also, let's require a user be passed into updateLastModified rather than fall back to the auth'd user.

No user passed in, updated_by gets nulled out.