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 
yes, it's definitely possible for `previous_state.mode == u"verified"`. This logic is supposed to work as follows for these two conditions:
1) user registers in a verified track. Instructor unenrolls them and enrolls them through instructor dash
2) user registers in a verified track. Instructor "re-enrolls" them through instructor dash

In case 2, we don't want the user's course mode to change (users were being downloaded to honor this way). In Case 1, we would want the user to be in "honor" when they're enrolled after being unenrolled.