Route for accepting an application invitation.

- Domain method for enabling an application role.
- Updated ApplicationRole model `history` property so that it serializes
  the `status` correctly

tests/routes/applications/test_invitations.py

@@ -0,0 +1,23 @@
+from flask import url_for
+
+from tests.factories import *
+
+
+def test_accept_application_invitation(client, user_session):
+    user = UserFactory.create()
+    application = ApplicationFactory.create()
+    app_role = ApplicationRoleFactory.create(application=application, user=user)
+    invite = ApplicationInvitationFactory.create(
+        role=app_role, user=user, inviter=application.portfolio.owner
+    )
+
+    user_session(user)
+    response = client.get(url_for("applications.accept_invitation", token=invite.token))
+
+    assert response.status_code == 302
+    expected_location = url_for(
+        "portfolios.show_portfolio",
+        portfolio_id=application.portfolio_id,
+        _external=True,
+    )
+    assert response.location == expected_location