fix open by default app and expose default app

[wkloucek]

needs: cs3org/cs3apis#157

needed because of owncloud/web#5970 (comment)

After this PR the /app/list endpoint will provide a default app (if configured):

    {
      "mime_type": "application/vnd.oasis.opendocument.text",
      "ext": "odt",
      "app_providers": [
        {
          "name": "Collabora",
          "icon": "https://www.collaboraoffice.com/wp-content/uploads/2019/01/CP-icon.png"
        }
      ],
      "name": "OpenDocument",
      "description": "OpenDocument text document",
      "allow_creation": true,
      "default_application": "Collabora"
    }

[glpatcern]

Looks good, clearly having the default semantically explicit is better than just relying on the sorting of the app providers, which was definitely temporary.

At this point we could actually remove the logic to keep the app providers list ordered.

[wkloucek]

@glpatcern I updated the CS3apis, can we get this in?

[glpatcern]

(did not merge yet because I'm not code owner - will check with @ishank011 also to update the CODEOWNERS rules for those subtrees)