In what surely is a peak time for Montreal culture, with festivals and marches everywhere, the city as a whole and the English-speaking progressive and artistic communities in particular suffered a major loss yesterday. After 27 years, the Montreal Mirror abruptly stopped publication. News came first from a press release by Sun Media, a division of the Mirror’s parent company Quebecor. Then the alt weekly’s site redirected to a message from the editors thanking readers and contributors and stating that…