Externalize i18n strings from app/views/shared/snippets/_header.html.haml

What does this MR do?

Externalize i18n strings from app/views/shared/snippets/_header.html.haml

Does this MR meet the acceptance criteria?

Conformity

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