Subversion export command should include file extension

Currently, ivle.database.ProjectSubmission.get_svn_export_command may return a command like this:

svn export --username lecturer -r3 'http://svn.ivle.localhost/users/studenta/ivle-102/phase1.html' studenta

For directory submissions, this is fine, but for file submissions, this will create a file with no file extension. In the above example, this creates a file called "studenta" which is an HTML document.

get_svn_export_command should be fixed so it preserves the file extension of the submission path. In the above example, it should return:

svn export --username lecturer -r3 'http://svn.ivle.localhost/users/studenta/ivle-102/phase1.html' studenta.html