Always proxy reports downloads

What does this MR do?

This fixes a regression introduced by support for new reports: syntax in merge request and security dashboard.

The frontend when downloading reports directly (it parses JSON) it has to request file through GitLab (not redirect to external storage). This is needed, as otherwise the CSP fails.

What are the relevant issue numbers?

None yet.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?