Added send email check when changing usernames

[sabaigbe]

Fix for error produced when changing usernames with the USER_ENABLE_EMAIL flag set to False

[2019-07-11 10:34:59,531] ERROR in app: Exception on /user/change-username [POST]
Traceback (most recent call last):
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask/app.py", line 2311, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask/app.py", line 1834, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask/app.py", line 1737, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask/_compat.py", line 36, in reraise
    raise value
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask/app.py", line 1832, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask/app.py", line 1818, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask_user/user_manager.py", line 390, in change_username_stub
    return self.change_username_view()
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask_user/decorators.py", line 58, in decorator
    return view_function(*args, **kwargs)
  File "/home/ubuntu/github/my_app/venv/lib/python3.5/site-packages/flask_user/user_manager__views.py", line 141, in change_username_view
    self.email_manager.send_username_changed_email(current_user)
AttributeError: 'UserManager' object has no attribute 'email_manager'

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@14d30bb). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #267   +/-   ##
=========================================
  Coverage          ?   88.55%           
=========================================
  Files             ?       25           
  Lines             ?     1451           
  Branches          ?        0           
=========================================
  Hits              ?     1285           
  Misses            ?      166           
  Partials          ?        0

Impacted Files	Coverage Δ
flask_user/user_manager__views.py	95.73% <100%> (ø)

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[and-semakin]

Thank you for your contribution @sabaigbe!