[7.0/trunk] hr_timesheet_sheet - It is possible to add a timesheet line to a confirmed or closed timesheet

[7.0] Tested on runbot
(Build 13550)
server (5007)
addons (9237)
web (3974)

[trunk] Tested on runbot
(Build 13534)
server (4900)
addons (8765)
web (3755)

How to reproduce:
Log in with demo user.

1st way to reproduce
Go in Human Ressources -> Time Tracking -> My Current Timesheet
Click on Timesheet button "Timesheet Activities"

Now create a timesheet line for an older and closed timesheet

Expected: Error raised
Result: You can save it

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2nd way to reproduce

Go in Human Ressources -> Time Tracking -> My Current Timesheet
Now Approve you current timesheet

Go in Human Ressources -> Time Tracking -> Timesheet Activities
(Not with button "Timesheet Activities" on timesheet in order to have no default sheet_id on the view)

Create a line for today (which belongs to the timesheet you just approved)

Expected: Error raised
Result: You can save it