fix reuploading of legacy getpersonas themes (bug 921214)

Some important theme bug fixes.

[test 1: updating a getpersonas-uploaded theme (header-only or footer-only)](https://github.com/ngokevin/zamboni/compare/legacytheme?expand=1#diff-368507455056cae0c97c26f00581b555R547)

```diff
+        """
+        STR the bug this test fixes:
+
+        - Reupload a legacy theme (/w footer == leg.png) legacy, header only.
+        - The header would get saved as 'pending_header.png'.
+        - The footer would get saved as 'footer.png'.
+        - On approving, it would see 'footer.png' !== 'leg.png'
+        - It run move_stored_file('footer.png', 'leg.png').
+        - But footer.png does not exist. BAM BUG.
+        """ 
```

[test 2: approving a getpersonas-uploaded theme doesn't update preview images correctly](https://github.com/ngokevin/zamboni/compare/legacytheme?expand=1#diff-e65341e2fcec962f7e0d0a2e41d7be4eR543)

```diff
+        """
+        Test updating themes that were submitted from GetPersonas.
+        STR the bug this test fixes:
+
+        - Reupload a legacy theme and approve it.
+        - On approving, it would make a preview image with the destination as
+         'preview.png' and 'icon.png', but legacy themes use
+         'preview_large.jpg' and 'preview_small.png'.
+        - Thus the preview images were not being updated, but the header/footer
+          images were.
+        """ 
```