Improve usage of `dag_pipeline_tab` and `filter_pipelines_search`

What does this MR do?

This does the following changes:

• Follows our practices to always scope feature flags https://docs.gitlab.com/ee/development/feature_flags/development.html
• Adds a scope to dag_pipeline_tab and filter_pipelines_search
• Fixes a missing DAG tab not being enabled by default

Conformity

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