filemanager: setup sea-orm with entity generation

[mmalenic]

To ease the use of writing sqlx queries, sea-orm-cli entity generation could be used to generate types for the database tables automatically. Since this is a cli tool, it could be run before compiling code, or potentially to integrate it better with cargo build, it could be run inside build.rs.

This would also help with the querying API in #109 and #110, as there wouldn't be as much of a need to define types for values returned by the database.