Add BaseContainerService

What does this MR do?

Currently BaseService accepts a project. However since wiki can now belong to group, we need to expand it to accept either a group or a project. This MR introduces a BaseContainerService, where the container can either be a project or a group. This will be used in subsequent MRs.

I used this opportunity to make BaseContainerService use keyword arguments all the way, which is more flexible for subclass customization.

To be unintrusive, the original BaseService will remain unchanged functionality wise, still being scoped by project. We can later migrate service classes out of BaseService.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.