URL encode needed values

To test upload a PDF file once with special characters (e.g. "This+Is+a+test.pdf") and one without (e.g. "ThisIsATest.pdf").

Then test the following test-cases:

- [x] Viewing a PDF file as logged-in user still works
- [x] Viewing a shared PDF as logged-in user still works
- [x] Viewing a public shared PDF as logged-in user still works
- [x] Viewing a public shared PDF as anonymous user still works
- [x] Viewing a public shared PDF in a folder still works

Fixes #11