fix: `static_alias` unnecessarily creates new content objects when used with versioning

[fsbraun]

This PR fixes a regression introduced in #165 which leads to the creation of a new AliasContent object for a language

• each time the page with the template tag is viewed
• if no published version of the alias content is available

The regression is triggered by using _default_manager once where _base_manager should have been used.

165 introduced a test but missed the regression. This PR adds a regression test.