The page visit requires an ajax request to get the notes. Without
waiting for that to complete, additional navigation results in a
socket error.

Example failure from the stub service:
```python
Traceback (most recent call last):
  File "/usr/lib/python2.7/SocketServer.py", line 295, in _handle_request_noblock
    self.process_request(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 321, in process_request
    self.finish_request(request, client_address)
  File "/usr/lib/python2.7/SocketServer.py", line 334, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python2.7/SocketServer.py", line 657, in __init__
    self.finish()
  File "/usr/lib/python2.7/SocketServer.py", line 716, in finish
    self.wfile.close()
  File "/usr/lib/python2.7/socket.py", line 283, in close
    self.flush()
  File "/usr/lib/python2.7/socket.py", line 307, in flush
    self._sock.sendall(view[write_offset:write_offset+buffer_size])
error: [Errno 32] Broken pipe
```

MA-2152
MA-2153

TNL-4164

TNL-4108

TNL-3910

TNL-3908

TNL-2172

Also, I replaced a number of tearDown methods with addCleanup instead.
And also remove some unneeded patch.stopall() calls.

TNL-1926

This eliminates some flakiness related to the loading of the notes page. It also
sets the 'no content' version of that page as a separate page object so testing
that case is more explicit.

TE-849

TNL-1924

Co-Authored-By: Jean-Michel Claus <jmc@edx.org>
Co-Authored-By: Brian Talbot <btalbot@edx.org>
Co-Authored-By: Tim Babych <tim@edx.org>
Co-Authored-By: Oleg Marshev <oleg@edx.org>
Co-Authored-By: Chris Rodriguez <crodriguez@edx.org>