File model has to save service and unique Id

[luceos]

Currently the File model is unaware where the file was saved. We have to start storing this information so we can delete these files later on.

Modify the database:

• Add column remote_id, string, nullable
• Add upload_method to phpdoc of File

Change logic to:

• Save upload_method to database
• Store the remote id to the database, eg for Imgur

[luceos]

Fixed in https://github.com/flagrow/upload/commit/69227038f71a2f36f66c8d3b0dcc4b405fc574f4