Add cover_image_file_id to API data

[BrendonKoz]

Adds to the response data from the API, allowing the exhibit record's cover image file id to be exposed.

[BrendonKoz]

Unfortunately when I made this pull request, I didn't realize my text editor auto-adjusted the whitespace. Regardless, the only addition is line 16 (of the amended file). What are the chances of this getting added to the original /models/Api/Exhibit.php file?

$representation['cover_image_file_id'] = $record->cover_image_file_id;