Fix anchor tag in flash message.

https://www.pivotaltracker.com/story/show/117513779

tests/app/main/views/test_accept_invite.py

@@ -287,7 +287,8 @@ def test_signed_in_existing_user_cannot_use_anothers_invite(app_,
             banner_contents = flash_banners[0].text.strip()
             assert "You’re signed in as test@user.gov.uk." in banner_contents
             assert "This invite is for another email address." in banner_contents
-            assert "<a href='/sign-out'>Sign out</a> and click the link again to accept this invite." in banner_contents
+            assert "{} and click the link again to accept this invite.".format(
+                url_for("main.sign_out", _external=True)) in banner_contents
             assert mock_accept_invite.call_count == 0