Fixes the 500 for custom apearance header logo and logo

What does this MR do?

Fixes the access to custom logos and header logos as defined in the "Appearance" admin section.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

It's a regression bug introduced by 43ff7386 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11022).

The Appearance model was not authorized anymore in the UploadsController. This lead to customized logo and header logo not to be shown anymore

The development of this bugfix is sponsored by @siemens (/cc @bufferoverflow)

Screenshots (if relevant)

before

after

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?