This pull request cleans up the red RFC list API. Specifically

• replaces the fake (always empty) errata list with a boolean has_errata flag that is true when the RFC document has the verified-errata tag in datatracker
• replaces the fake (always ["txt", "xml"]) formats list with a structured list of available formats and the name of the blob for each format.
• eliminates the see_also field

The formats section of the response is derived from the StoredObject table. It lists all available formats the datatracker has, including json and notprepped. If some of these shouldn't be included in red's output, that decision belongs to red and it should filter them there. It now includes the name of the blob so that red does not have to hard-code the naming convention that datatracker uses.

The see_also field will not be used. Subseries memberships are conveyed through the subseries value already.

Red should link to the correct errata site page for a URL when has_errata is true.

For reference, this is the formats section of the updated API for an example RFC:

      "formats": [
        {
          "fmt": "xml",
          "name": "xml/rfc10101.xml"
        },
        {
          "fmt": "txt",
          "name": "txt/rfc10101.txt"
        },
        {
          "fmt": "html",
          "name": "html/rfc10101.html"
        },
        {
          "fmt": "pdf",
          "name": "pdf/rfc10101.pdf"
        },
        {
          "fmt": "json",
          "name": "json/rfc10101.json"
        },
        {
          "fmt": "notprepped",
          "name": "notprepped/rfc10101.notprepped.xml"
        }
      ],