Refactor Integration propagation services

What does this MR do and why?

This MR refactors the service classes that propagate integrations in preparation for more work in #391526 (closed). It:

• Namespaces services into Integrations::Propagation::
• Removes IntegrationList and DataList models (the main change for #391526 (closed) because it seems easier to map data using methods than add another class to support the GitLab for Slack app data)

There should be no functional changes.

#391526 (closed)

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Related to #391526 (closed)