Allow site admins to disallow student access to Quickmail

* Added new config setting to allow site admins to disallow student access to Quickmail
* Added behat tests

I added a new config setting for block_quickmail_allowstudents called "Never (-1)". If it is set, then it will not show the config setting to "Allow students to use Quickmail" in the block configuration and will override any previously set value for the setting.

I also added a related Behat test to ensure that the feature is working.