DATABASE.md: Add image_links column

This solves the problem of rows which don't need a full cut video,
but we'd like to link to an image or a short gif or clip of it.
It is a sheet input that is only used in the output sheet, so it doesn't affect the wubloader itself.

DATABASE.md

@@ -130,6 +130,7 @@ columns                    | type                               | role        |
 `description`              | `TEXT NOT NULL DEFAULT ''`         | sheet input | Event description. Provides the default title and description for editors, and displayed on the public sheet.
 `submitter_winner`         | `TEXT NOT NULL DEFAULT ''`         | sheet input | A column detailing challenge submitter, auction winner, or other "associated person" data. This shouldn't be relied on in any processing but should be displayed on the public sheet.
 `poster_moment`            | `BOOLEAN NOT NULL DEFAULT FALSE`   | sheet input | Whether or not the event was featured on the poster. Used for building the postermap and also displayed on the public sheet.
+`image_links`               | `TEXT[] NOT NULL`                 | sheet input | Any additional gif or image links associated with the event. Displayed on the public sheet.
 `notes`                    | `TEXT NOT NULL DEFAULT ''`         | sheet input | Private notes on this event, used eg. to leave messages or special instructions. Displayed to the editor during editing, but otherwise unused.
 `allow_holes`              | `BOOLEAN NOT NULL DEFAULT FALSE`   | sheet input | If false, any missing segments encountered while cutting will cause the cut to fail. Setting this to true should be done by an operator to indicate that holes are expected in this range. It is also the operator's responsibility to ensure that all allowed cutters have all segments that they can get, since there is no guarentee that only the cutter with the least missing segments will get the cut job.
 `uploader_whitelist`       | `TEXT[]`                           | sheet input | List of uploaders which are allowed to cut this entry, or NULL to indicate no restriction. This is useful if you are allowing holes and the amount of missing data differs between nodes (this shouldn't happen - this would mean replication is also failing), or if an operator is investigating a problem with a specific node.