Migrate to material-design-icons. Closes #116.

biscuit/core/settings.py

biscuit/core/templates/core/base.html

biscuit/core/templates/core/crud_events_ul.html

biscuit/core/templates/core/footer-menu.html

biscuit/core/templates/core/group_full.html

biscuit/core/templates/core/groups.html

biscuit/core/templates/core/person_full.html

biscuit/core/templates/core/system_status.html

biscuit/core/templates/core/turnable.html