support joining a team

common/test/acceptance/pages/lms/teams.py

common/test/acceptance/tests/lms/test_teams.py

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

lms/djangoapps/teams/static/teams/js/views/team_join.js

lms/djangoapps/teams/static/teams/js/views/team_profile.js

lms/djangoapps/teams/static/teams/js/views/teams_tab.js

...ngoapps/teams/static/teams/templates/team-join.underscore

lms/static/js/components/header/views/header.js

lms/static/js/spec/main.js

lms/static/sass/views/_teams.scss

lms/templates/components/header/header.underscore