permissions: Apply permissions when files move and directories are created

### Problem

I run beets as myself, where I have umask set to 027. That's restricted, so I use the permissions plugin to set broader permissions for my media files.

After running `mbsync`, paths might be rewritten, so new directories are being created and they get the permissions by umask. However, the permissions plugin is only run after imports.

I am aware that I could solve this issue by other means, but maybe the documentation could point this out more verbose or the plugin could be expanded.

### Setup

* OS: Debian testing
* Python version: 3.8.2
* beets version: 1.4.9
* Turning off plugins made problem go away (yes/no): n/a

My configuration for the permissions plugin is:

```yaml
permissions:
file: 644
dir: 755
```