Folder structure for managing documents and images

### As a Joplin content editor, I want to be able to organize documents and images into groups, so that I can easily find my document later.

**Is your feature request related to a problem? Please describe.**
We're going to have the ability to host documents on Joplin. That could get unwieldy. We need to be able to set up something like a file folder structure so that users can organize their docs.

**Describe the solution you'd like**
File folder structure, maybe

**Describe alternatives you've considered**

**Additional context**