[8.0] [FIX] hr_timesheet_sheet: hide "Add a Line" button if a timesheet is not editable

A user can see the button "Add a line" in a timesheet even if he cannot edit the timesheet lines.

This pull-request hide the button when the widget is in read-only.

Scenario:
As admin on runbot,
1/ Go to "Human Resouces" / "Time Tracking" / "My Current Timesheet"
2/ Create a new Timesheet & save
3/ Submit the timesheet to manager & edit
4/ The button "Add a Line" is visible, but the timesheet line are not editable