Update UserCourseTag model to have modified and created timestamps.

This is for ECOM-525, sub-task ECOM-682 to update the UserCourseTag model to have a modified and created timestamp. Added tests to ensure they are updated as expected with interactions for the models. Cleaned up another test in the file while I was in there. 

@rlucioni @wedaly @dianakhuang 

Re: UserCourseTag model @shnayder I was curious if there was a specific desire to use this model for the "Email Opt-In" setting versus creating a new UserOrgTag Model.  As I understand it, we want to use theUserCourseTag model so we could audit which course a user enrolled in when they decided to opt in/out of emails.  Otherwise, I just wanted to check to see if it's possible we want a UserOrgTag Model instead, which links a user preference to an organization. 
@cpennington When making this change to the UserCourseTag model, the migration script added this line to change the varchar course_id field to a CourseKeyField. Lining up the changes to the models with the migration scripts for the project, it looks like the model was changed, but a migration script was not created to reflect the change. Should I leave this line in, or is it alright for their to be a difference between the migration scripts and the model?