Added code to subscribe the user to the task when he comments on the task

If he is already subscribed we dont do anything regarding the
subscription.
When task status is changed, we send an notification

We also store the date on which the task was completed as well as who
closed the task.
Changing the header text of Tasks widget in dashboard
Resetting metadata after pre and before post upgrade.
Removing the column description from task table.
Adding columns hours_spent and comments to task table.
Calculating the number of comments and hours spent for old data.