fix syntax warnings in reverse_cron

[apollo13]

regular expressions should use raw strings, otherwise backslashes would have to be escaped. This is nowadays a warning (SyntaxWarning: invalid escape sequence '\D'), but will turn into an error in the future.

This is what I see on the console without using raw strings:

/app/share/devtoolbox/devtoolbox/views/reverse_cron.py:323: SyntaxWarning: invalid escape sequence '\D'
  starting_day = re.sub("\D", "", self._day_every_x_starting_dropdown.get_selected_item().get_string())
/app/share/devtoolbox/devtoolbox/views/reverse_cron.py:324: SyntaxWarning: invalid escape sequence '\D'
  interval = re.sub("\D", "", self._day_every_x_dropdown.get_selected_item().get_string())
/app/share/devtoolbox/devtoolbox/views/reverse_cron.py:336: SyntaxWarning: invalid escape sequence '\D'
  x = re.sub("\D", "", self._day_range_x_dropdown.get_selected_item().get_string())
/app/share/devtoolbox/devtoolbox/views/reverse_cron.py:337: SyntaxWarning: invalid escape sequence '\D'
  y = re.sub("\D", "", self._day_range_y_dropdown.get_selected_item().get_string())