Fix displaying wrong membership on "My Team" view.

The heart of this fix is to change the "My Team" view to use a
collection of teams instead of memberships. The team card is
refactored to only take a team, rather than attempt to be polymorphic
over teams and team memberships. This change enabled removing a good
amount of old code. This also requires adding a `username` parameter
to the teams list endpoint which allows getting a list of all teams
for a single user, in order to allow refreshing the "My Teams" view
correctly. Currently this list is of length at most one.

TNL-3410

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

@@ -46,6 +46,11 @@ class TeamCardsMixin(object):
         """Return the names of each team on the page."""
         return self.q(css=self._bounded_selector('p.card-description')).map(lambda e: e.text).results
 
+    @property
+    def team_memberships(self):
+        """Return the team memberships text for each card on the page."""
+        return self.q(css=self._bounded_selector('.member-count')).map(lambda e: e.text).results
+
 
 class BreadcrumbsMixin(object):
     """Provides common operations on teams page breadcrumb links."""

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

@@ -78,6 +78,18 @@ class TeamsTabBase(EventsTestMixin, UniqueCourseTest):
         self.assertEqual(response.status_code, 200)
         return json.loads(response.text)
 
+    def create_memberships(self, num_memberships, team_id):
+        """Create `num_memberships` users and assign them to `team_id`. The
+        last user created becomes the current user."""
+        memberships = []
+        for __ in xrange(num_memberships):
+            user_info = AutoAuthPage(self.browser, course_id=self.course_id).visit().user_info
+            memberships.append(user_info)
+            self.create_membership(user_info['username'], team_id)
+        #pylint: disable=attribute-defined-outside-init
+        self.user_info = memberships[-1]
+        return memberships
+
     def create_membership(self, username, team_id):
         """Assign `username` to `team_id`."""
         response = self.course_fixture.session.post(
@@ -339,6 +351,18 @@ class MyTeamsTest(TeamsTabBase):
             self.my_teams_page.visit()
         self.verify_teams(self.my_teams_page, teams)
 
+    def test_multiple_team_members(self):
+        """
+        Scenario: Visiting the My Teams page when user is a member of a team should display the teams.
+        Given I am a member of a team with multiple members
+        When I visit the My Teams page
+        Then I should see the correct number of team members on my membership
+        """
+        teams = self.create_teams(self.topic, 1)
+        self.create_memberships(4, teams[0]['id'])
+        self.my_teams_page.visit()
+        self.assertEqual(self.my_teams_page.team_memberships[0], '4 / 10 Members')
+
 
 @attr('shard_5')
 @ddt.ddt