Rework tables that store uploads to contain the object key

[ThisIsMissEm]

Currently when we process uploads or receive incoming media we only store a url for that media.

So for instance for accounts, we store:

• avatar_url
• cover_url

For media, we store:

• type
• url
• width
• height
• thumbnail_type
• thumbnail_url
• thumbnail_width
• thumbnail_height

For custom emojis, we store:

• url

Given these properties, it's not possible to know whether the media is stored in our uploads (s3 / fs) or whether the media comes from a remote source. Additionally, if it is stored in our uploads, we don't store the key for the file, which is necessary when performing deletions of uploaded files.

We probably want to move towards a schema like:

• avatar_key (nullable)
• avatar_remote_url (nullable)

Where _key refers to the file in our storage, and _remote_url is for media stored on other servers.

[ThisIsMissEm]

We may additionally also want to store the blurhash of media