Add a context property to downloads

[MortenHofft]

When doing downloads for the B-cube project the context of the cube is lost when arriving at the download page. At this point it is just a large SQL statement. To ensure a smoother usability transition from the UI Cube builder to the download page we would like to provide some context.

So the idea is that when creating a download we add the option to provide some free text context.

{
"context": "Something to describe that it is a cube download and the parameters used",
"creator": "gbif_username",
  "sendNotification": true,
  "notification_address": [
    "gbif@example.org"
  ],
  "format": "DWCA",
  "predicate": { ....
}

On the download pages ew can then show the context to help explain the SQL and how it came about.

See gbif/portal16#1978