Migrate existing Generic Alert Endpoints to HTTP integrations format

What does this MR do?

This adds a data migration to migrate/copy instances of AlertService over to the new HttpIntegration model.

This new model is used as part of &4442 (closed).

There is a down method included to undo the added HttpIntegration models. This allows us to migrate & rollback as needed.

database Query:

• Database-lab: https://gitlab.slack.com/archives/CLJMDRD8C/p1603766823244700
• Explain: https://explain.depesz.com/s/cGOK

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #255519 (closed)