History Tracking

Created by: ianwall

If access to timecard functions is restricted to say developers and above via the thresholds on the config page, anyone can still see the estimates and time spent in the Issue History. We are using this for internal estimating and don't want customers who submit issues to see that. Hence I've added a config option to enable/disable tracking changes to time estimates and time spent. I couldn't find a way to disable tracking on a plugin by plugin basis based on access level, so I've done it this way. Note tracking defaults to off.

Also fixed a problem in log_time.php: now uses auth_get_current_user_id() rather than auth_get_current_user_cookie().