[BUG] helpdesk_mgmt_timesheet: timesheet with projects without allow_timesheets

[DaniilDigtyar]

Raise error trying to create a timesheet (account.analytic.line) in a helpdesk.ticket configured with a project with allow_timesheets disabled.

helpdesk_mgmt_timesheet

It can be also be affected by helpdesk_mgmt_project

Describe the bug

When creating a timesheet in a ticket with a project with allow_timesheets disabled it raises an error "At least one analytic account must be set" causing confusion to the user.
It should show a more detailed error message.

To Reproduce

• Create a project with allow_timesheets disabled
• Create a ticket with that project
• On create a timesheet for that ticket, it raises the error At least one analytic account must be set

Affected versions:

• Tried only on 18.0 OCB

Expected behavior
On create a timesheet for a project that it has allow_timesheets disabled, to show a more explicit error message like "This project doesn't allow timesheets to be registered.".