Fix unbound local variable error in section console

[c-w]

Issue found by @taupalosaurus

Full stacktrace:

[2018-07-03 16:54:18,984] ERROR in app: Exception on /admin/course/1/section/ [GET]
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.5/site-packages/flask_restful/__init__.py", line 271, in error_router
    return original_handler(e)
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1518, in handle_user_exception
    return handler(e)
  File "/usr/local/lib/python3.5/site-packages/applicationinsights/flask/ext.py", line 167, in exception_handler
    raise exception
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.5/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.5/site-packages/flask_login/utils.py", line 228, in decorated_view
    return func(*args, **kwargs)
  File "/code/server/controllers/admin.py", line 56, in wrapper
    return func(*args, **kwargs)
  File "/code/server/controllers/admin.py", line 494, in section_console
    enrollments=enrollments, staff=staff, emails=student_emails_str)
UnboundLocalError: local variable 'enrollments' referenced before assignment

In order to make the fix easier, I first did some refactoring/cleanup in 380d88f. The actual fix is in 2d102b6.

[colinschoen]

LGTM

Thanks.