Fix selenium integration tests

[tompollard]

The integration tests arbitrarily fail, possibly due to timing issues (e.g. looking for an element in a webpage before it is fully loaded). Some example errors are:

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ERROR: test_publish (project.test_browser.TestSubmit)
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Traceback (most recent call last):
  File "/root/src/github.com/MIT-LCP/physionet-build/physionet-django/project/test_browser.py", line 290, in test_publish
    'id_editor')).select_by_visible_text('admin')
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/support/select.py", line 120, in select_by_visible_text
    self._setSelected(opt)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/support/select.py", line 212, in _setSelected
    option.click()
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/webelement.py", line 80, in click
    self._execute(Command.CLICK_ELEMENT)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/webelement.py", line 633, in _execute
    return self._parent.execute(command, params)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 321, in execute
    self.error_handler.check_response(response)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/errorhandler.py", line 242, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.ElementNotInteractableException: Message: Element <option> could not be scrolled into view


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ERROR: test_publish (project.test_browser.TestSubmit)
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Traceback (most recent call last):
  File "/root/src/github.com/MIT-LCP/physionet-build/physionet-django/project/test_browser.py", line 278, in test_publish
    self.selenium_login(username='admin', password='Tester11!', new=True)
  File "/root/src/github.com/MIT-LCP/physionet-build/physionet-django/project/test_browser.py", line 97, in selenium_login
    element.click()
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/webelement.py", line 80, in click
    self._execute(Command.CLICK_ELEMENT)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/webelement.py", line 633, in _execute
    return self._parent.execute(command, params)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 321, in execute
    self.error_handler.check_response(response)
  File "/root/venv/3.6/lib/python3.6/site-packages/selenium/webdriver/remote/errorhandler.py", line 242, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.StaleElementReferenceException: Message: The element reference of <a id="nav_account_dropdown" class="nav-link dropdown-toggle" href="#"> is stale; either the element is no longer attached to the DOM, it is not in the current frame context, or the document has been refreshed


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