Command exits with non-zero status if changes without migrations exist.

Thanks Nick Pope for the initial patch.

Thanks gjanee@ucop.edu for the report and Tim Graham for the review.

Traceback (most recent call last):
  File "tests\admin_scripts\tests.py", line 646, in test_startapp_unicode_name
    content = f.read()
  File "lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x81 in position 46: character maps to <undefined>

Fixes a regression from 2bd1bbc4. Thanks Daniel Wiesmann for the report
and Tim Graham for the review.

Refs #24622 -- Documented alternatives to some test response attributes when using alternative template engines.

Used request.META['SCRIPT_NAME'] as the site_url if it hasn't been
customized from the default value of '/'.

Added instructions for installing virtualenv and virtualenvwrapper.

If the model form had a form field specified, the label rendered
as "None".

The table body (tbody) was shifted by one column to the left
if the first inline form field is hidden.

Thanks to Markus for the suggestion and Tim for the review.

Thanks to Andriy Sokolovskiy for the report and Tim Graham for the review.