Support multiple enclosures/attachments

[Eitot]

Some decisions have to be made to support this.

1. How to store additional enclosure/attachment URLs

• Use the "enclosures" column of the "messages" table and store multiple URLs, separated by a delimiter. This option would require no changes to the database, but it would not be backwards compatible.
• Add a new column to the "messages" table and migrate existing data, optionally keeping the "enclosures" column.
• Add a new table and link the "messages" table to it, , optionally keeping the "enclosures" column.

2. How to show the enclosures in the enclosure bar of the article viewer

• Show file icons/previews and links for each enclosure/attachment
• Change the enclosure bar so that it shows a pop-up menu instead, similar to Mail:
  

3. What to do with the "Enclosure URL" column in the article list

4. What to do with the `$ArticleEnclosureFilename$` and `$ArticleEnclosureLink$` tags in article styles

Any ideas?