use textual css classes for issue priorities

textual css classes for issue priorities were introduced by redmine/redmine@113f8b5cf278ff32bd55bf63aafe11c3f5a085f1

For available textual css classes run this in your redmine database (column position_name):
```SQL
SELECT * FROM `enumerations` WHERE `type` = 'IssuePriority'
```

Edit: and also merge the original A1 v1.1.2