Add placeholder instance integration models

What does this MR do and why?

This MR adds placeholder models for the instance integrations in order to populate them in future MRs. Example MR: !177125 (merged)

It's done in bulk because individual MRs like !177125 (merged) require updating db/docs/instance_integrations.yml which triggers database review just for one yml file. The MRs themselves don't have any db changes, they extract functionality from regular integration model into a concern and share the concern between two integration models, regular and instance.

This way we can do db review once and skip doing reviewer/maintainer db reviews for individual model conversions which saves up a lot of time, since there are 40+ integrations to convert.

Mentions #474809 (closed)

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