Relax permissions where applicable

[aaronleopold]

There are a decent amount of API endpoints that have yet to be updated according to the user permissions system. These are still restricting to the server owner, only. For example, replacing a book thumbnail. This is functionality that can probably be granted to those with UserPermission::ManageLibrary.

This issue is just a reminder to, as I go through and develop other features, try and touch up any endpoints as I go.