Add feature spec for Release page MVC

What does this MR do?

This MR adds feature spec for release page MVC and fixes a small bug on frontend

Screen shot

It shows (external source) correctly.

Captured from Selenium (with Capybara) so that the appearance of the page is a bit different from the actual one.

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/41766

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary => It's behind a feature flag.
• [-] Documentation created/updated => Handled in a separate MR
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer