Add target response time alarm

[aaronxsu]

After a discussion with @BryanQuigley regarding changes and monitored metrics in this issue: https://github.com/PublicMapping/districtbuilder/issues/1277 , we find the following helpful before the next release:

• add the target response time alarm in cloudwatch with terraform
• update the lambda function for the alarm bot to include this alarm
• bonus point: only create alarms for production, conditionally