Non square boards would break scraper

[climu]

We should change have a way to check if board size is square. At the moment a non square board would raise such:

  File "/openstudyroom/league/models.py", line 521, in parse
    prop = utils.parse_sgf_string(self.sgf_text)
  File "/openstudyroom/league/utils.py", line 175, in parse_sgf_string
    out['board_size'] = int(out['board_size'])
ValueError: invalid literal for int() with base 10: '19:9'