Can't correct ".po" template name: "There is already a file in the given path."

[Jeroen T. Vermeulen]

Scenario:
 * A product series has a template with path foo.pot.
 * A new template is uploaded, but it's called foo.po instead of foo.pot.
 * The queue reviewer sets the upload's type to template and fixes the path.
 -> The approval form rejects the upload's new path, saying "There is already a file in the given path."

The problematic code is in browser/translationimportqueue.py, in _validatePath (the comment says it checks for an IPOFile in the given path, but in this case it's been checking for a template AFAICT). The form should accept the change if the new data for the entry produces a complete match with the existing file. In that case the upload should be attached to the existing file.