Make the new header app for Repository, Blob and Overview default

The new Vue app for header of Repository, Blob and Project Overview pages is temporarily hidden behind ?common_repository_blob_header_app=true URL param. This issue is extracted from #497051 (closed) as sort of a roll-our issue.

The steps we need to take:

1. Remove the condition that renders the new Vue app, when the param is present.
2. Run all feature and E2E tests for the three views that are affected. There might be selectors that will need adjusting.
3. Investigate any failing tests that are not cause by the selector mismatch. Make sure all of the old functionality is available in the new version.
4. Manual test the new view for CE and EE.
5. Remove any HAML file that is no longer in use, but was rendered as part of the old header area.