devilry_theme: Made a css class for links that open a new window.

See #351.
devilry_subjectadmin...addgroups: Mark new-window urls.

See #351.
devilry_subjectadmin...managestudents: Mark new-window urls.

See #351.
devilry_theme: Added new-window-link and email icons (#351).
devilry_theme: Add icon to all mailto links.

Fixes #351.