A tune up for the Coursera creds submission process

The focus of this pull request is that swirl now confirms the user's Coursera creds before submitting anything. Course ID is also stashed alongside the email and submission password in the `credfile`.

This also fixes a bug that I discovered in our naming of the `credfile`. 

Previously, we constructed the path as follows: 
```
credfile <- file.path(e$udat, paste0(e$les$course_name,".txt"))
```

`e$les$course_name` is `NULL`, since `course_name` is actually an *attribute* of `e$les`. So, the result was that a file called `.txt`, which is a hidden file, was created in the `user_data` directory. No wonder people were having such a hard time finding it if they wanted to change something!

**Please play around with this pull request and make sure everything is working correctly before we merge!**