fix enrollment issue in instructor dash (ECOM-776)

Fixes an issue introduced in #5913 where if a verified student is unenrolled, then is reenrolled through the instructor dash, they are reenrolled as verified. They should be reenrolled as honor.

https://openedx.atlassian.net/browse/ECOM-776

@dianakhuang 
@rlucioni 
Is it possible for `previous_state.mode` to equal `u"verified"`? If so, should this student be enrolled as honor or verified?