Settings file in each project for defining a logo

[lourot]

At the moment in the profiles, only repos in an org have a logo.

For each repo without a logo, we want to see a [+] icon being a link to a document explaining that:

you can define a logo for brillout/awesome-redux by committing .ghuser.io.json into brillout/awesome-redux :

{
  "_comment": "Settings for ghuser.io. See <link to example/doc>",
  "avatar_url": "https://..."
}

NOTE: This is a per-project file, not per-user, do not mix up with #12

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db/fetch.js should fetch these settings and add them to db/db.json so that the frontend can make use of them.