EFCore improvements and cleanups

[ferferga]

This PR cherrypicks some of the changes made in #6049 in order to ease the review:

• Remove all the unused usings in the solution (we might want to enforce not unused usings to avoid having this situation again)
• Logs EFcore migrations, so users are aware of them
• Uses appHost instead of creating a new context in Program.cs
• (Not in EFCore Image model update and cleanup #6049) Decouples model configuration into its own separate classes, so they're easier to maintain and doesn't clog the JellyfinDb.cs file