GNU social

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# Attachments and Links
## Terminology
- ** Attachment **: Anything that can be attached to a note
- ** Link **: Anything remote linked to in a note
- ** AttachmentThumbnail **: The thumbnail of an attachment

Not every Link has an attachment. An attachment is created for a link when it becomes something attachable. So happens when additional information on a link's URL becomes available.

Following how it is done with Links, anything with a thumbnail in GNU social must have an attachment as only attachments can have a thumbnail.

This design prevents file and thumbnail duplication, thus saving storage.

An example of a module making use of this system is the Avatar component. It creates an attachment and uses the thumbnail route to access different avatar sizes via the `AttachmentThumbnail` controller.

# Migrating data from v2

## File to Attachment

Direct transformation:
* `id`       => `id`
* `filehash` => `filehash`
* `mimetype` => `mimetype`
* `size`     => `size`
* `modified` => `modified`

Requires computation:
* `filename` => {if file is in storage then `filename`, else null}
* `width` => {if file is known by V3, re-compute then `width`, else null}
* `height` => {if file is known by V3, re-compute then `height`, else null}
* Count references and fill `lives`

## File to Link

**Only if `url` and `urlhash` are not null.**

Direct transformation:
* `id`      => `id`
* `url`     => `url`
* `urlhash` => `url_hash`

## File and File_to_post to AttachmentToNote

Direct transformation by joining File f and File_to_post fp on `id` = `file_id` with fp.file_id, fp.post_id, f.title, f.filename, fp.modified:
* `file_id`  => `attachment_id`
* `post_id`  => `note_id`
* `modified` => `modified`

Requires computation:
* `title` ?? `filename`  => `title`

## File_thumbnail to AttachmentThumbnail

**For every attachment where we have the original (filename not null), just let them be regenerated on request.** For all the others, compute the mime-type and rescale to the new default sizes.

The field `url` from File_thumbnail has duplicated information that was already migrated in the tables above and can, therefore, be safely ignored now.

# Embed and StoreRemoteMedia
// TODO
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