TNL-549: Fix thread editing.

**Ticket:** [TNL-549](https://openedx.atlassian.net/browse/TNL-549)

@jimabramson , @tymofij, @cahrens  please review.
`get_discussion_category_map` returns something like that:
```python
{
    'subcategories': {
        'Week 1': {
            'subcategories': {}, 
            'children': [u'Inline Cohorted Topic'], 
            'entries': {
                u'Inline Cohorted Topic': {
                    'sort_key': None, 
                    'is_cohorted': False, 
                    'id': u'ef5adaa134fa49a0b71fbbcf4dc85f2b'
                }
            }
        }
    }, 
    'children': [u'Cohorted Topic', u'General', 'Week 1'], 
    'entries': {
        u'General': {
            'sort_key': u'General', 
            'is_cohorted': False, 
            'id': u'i4x-jsa-101-course-x'
        }, 
        u'Cohorted Topic': {
            'sort_key': u'Cohorted Topic', 
            'is_cohorted': False, 
            'id': u'cohorted_topic'
        }
    }
}
```
`get_discussion_categories_ids` returns just a list of ids:
```python
[u'ef5adaa134fa49a0b71fbbcf4dc85f2b', u'i4x-jsa-101-course-x', u'cohorted_topic']
```