Cache team_size on CourseTeam model and remove is_active field.

lms/djangoapps/teams/errors.py

lms/djangoapps/teams/migrations/0006_add_team_size.py

...s/migrations/0007_auto__del_field_courseteam_is_active.py

lms/djangoapps/teams/models.py

lms/djangoapps/teams/serializers.py

lms/djangoapps/teams/static/teams/js/models/team.js

...goapps/teams/static/teams/js/spec/views/edit_team_spec.js

...s/teams/static/teams/js/spec_helpers/team_spec_helpers.js

lms/djangoapps/teams/tests/test_models.py

lms/djangoapps/teams/tests/test_views.py

lms/djangoapps/teams/views.py