[Task]: Include seed file size in org dashboard

[SuaYoo]

Description

The "Storage" dashboard panel should also include the total seed file size.

Context

No response

[tw4l]

Seed files and collection thumbnails are added to the org stats in #2710, so should be ready for this. A migration is also added to make sure collection thumbnail storage is counted in the total (and provide a separate number for that in the API response)