Simplify and speed up slack rendering

[Konstantinov-Innokentii]

What this PR does

This PR simplify and speed up slack rendering.

For alert group with large amount of alerts query self.alert_group.alerts.filter(is_resolve_signal=False) took a several seconds to execute, since is_resolve_signalnot indexed. On the other hand, that query makes no sence, because anyway we using all alerts, when rendering their count in slack:

 def _get_text_alert_grouped(self):
        alert_count = self.alert_group.alerts.count()
        link = self.alert_group.web_link

        text = (
            f":package: Showing the last alert only out of {alert_count} total. "
            f"Visit <{link}|the plugin page> to see them all."
        )

        return text

So, I decided to remove filtering by is_resolve_signal in that case and leave only one alert_group.alerts.count() query.

Also, I checked that we have no any organizations which installed slack not via granular permissions and removed this check, which produced 4 SELECTs. For OSS slack we provide app manifest, so self.alert_group.channel.organization.slack_team_identity.installed_via_granular_permissions check can be safely removed.

[vstpme]

LGTM