[MS Teams] Use alert_subject as ms_teams_alert_summary unless set otherwise

[iamxeph]

Description

ms_teams_alert_summary is actually being used in MS Teams for notification title, so it is better to use alert_subject as its default.

For backward compatibility, this PR changes its default to None, and in the alerter code, if ms_teams_alert_summary is None, it will use alert_subject as its value instead.

Checklist

• I have reviewed the contributing guidelines.
• I have included unit tests for my changes or additions.
• I have successfully run make test-docker with my changes.
• I have manually tested all relevant modes of the change in this PR.
• I have updated the documentation.
• I have updated the changelog.

Questions or Comments

[jertel]

Thanks for your contribution to make ElastAlert 2 better!