Description of problem:
  Project Version with null projectType allows PO upload/download, which it shouldn't.

Version-Release number of selected component (if applicable):
3.1-SNAPSHOT (20131003-1234)

How reproducible:
100% (except the 500 error)

Steps to Reproduce:
1. Define a project with a non-null projectType
2. Define a project version with a null projectType
3. Upload a source document
4. Attempt to download an "offline PO" via the documents page for a locale.
5. Attempt to upload a PO file via the documents page for a locale.
6. Attempt to download an "offline PO" via the editor.
7. Attempt to upload a PO file via the editor.

Actual results:
At step 4, the ".po" link is disabled (as it should be), but at steps 5, 6, and 7 the action is allowed.  In some cases, the upload can lead to a 500 server error (unable to reproduce).

Expected results:
Steps 4, 5, 6 and 7 should be prevented (UI elements should be disabled), and the server should also have its own checks in case of rogue/buggy clients.

Additional info:
Where possible, we must check for null project version type both in the UI and in the corresponding REST services, both for upload and download.