8.0 'project_issue' fix priority field migration (onesteinbv)



Port 'project_issue' modification realized by OneStein BV. (branch onesteinbv:8.0-os)

These are old values:
`('1', 'Highest'), ('2', 'High'), ('3', 'Normal'), ('4', 'Low'), ('5', 'Lowest')`
and these are the new ones:
`('0', 'Low'), ('1', 'Normal'), ('2', 'High')`
so I don't think we should change this. Previous mapping is correct.