[server] Support for storing preview files

[ua741]

Description

• This change introduced the concept of associated object for a file.
• Added additional columns for object_id, object_nonce, and object_size.

Depending upon data_type, the values of certain columns will be nil.
The original size column will reflect total size for that particular type. In case of vid_preview, it's size of the playlist + size of the preview video.

Tests

• Replication
• Test Deletion post replication

[mnvr]

Left comments on some trivial typos etc, also the file render.svg seems unrelated to this PR.