Customizable favicon

What does this MR do?

Allows changing the default favicon to a custom icon. The CI status favicons are generated from the original image as well.

• Upload custom favicon in appearance settings
• Main favicon is replaced by the custom one
• All CI status favicons are generated from the uploaded image

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

Why was this MR needed?

GitLab already allows to customize the login screen and the header logo. Allowing to customize the favicon as well greatly helps differentiating the different GitLab instances.

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

Screenshots (if relevant)

Appearance settings

CI status icon

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15661