Improve discoverability of email functionality

[donnapep]

Is your feature request related to a problem? Please describe

The email feature is too hidden in Sensei's settings, making it difficult to find.

Describe the solution you'd like

Move the email feature out of settings. Instead, add an Emails submenu between Grading and Messages.

We will also need to disallow the ability to add new emails, as we don't offer that functionality currently (i.e. no New Email button).