added pins_get to get channel pins

[webtax-gh]

added pins_get to get channel pins

example test code:

bot.pins_get(event.msg->channel_id, [log, &bot](const dpp::confirmation_callback_t& completion) {
    dpp::message_map pins = std::get<dpp::message_map>(completion.value);
    log->debug("Pins got! number of pins={} http status={}", std::distance(std::begin(pins), std::end(pins)), completion.http_info.status);
});

this PR also includes fixes to channel_edit_permissions and channel_invites_get, as they were using the /api/channel endpoint instead of /api/channels

pins_get has been tested, the fixes to channel_edit_permissions and channel_invites_get have not been tested.