Email hostname field can be confusing when a default is provided.

Problem/Motivation

On the CAS settings page, the CAS setting "Automatically register users" opens additional registration options including a field for a default email hostname.

When the email hostname is configured with a default, it nicely populates on the Bulk Add CAS Users form even if the "Automatically register users" setting is unchecked and the options are hidden (including email hostname).

On the Bulk Add CAS Users page, the default email hostname field can be confusing for user admins who see the editable field and think of the provided default as a prompt to fill it in with the new user's email address. I have had cases where admins enter full email addresses into the email hostname field which then creates malformed emails for the accounts.

Steps to reproduce

1. On the CAS settings page, checkmark "Automatically register users."
2. In the Email hostname field, add a default hostname (e.g., test.com) and save.
3. On the Bulk Add CAS Users page, enter a CAS username and edit the email hostname field to be a full email address, which corresponds to cases of user confusion, then save.
4. View and edit the new user and see their email is malformed.

Proposed resolution

On the Bulk Add CAS Users page, when a default email hostname is provided from CAS settings, change the field to be disabled or readonly.