(no-ticket) Fix some things not working with remote filetracker

Committer's note:

I'm pretty sure the Tadeusz is the author of those changes,
so I set him as author, but I found the changes uncommitted,
so I'm not 100% sure it's his.

 -- Wojciech Dubiel

Change-Id: I70a51c2e1212cda3e206c0d86f578bdb6c379b86

oioioi/confirmations/utils.py

@@ -42,10 +42,10 @@ def submission_receipt_proof(submission):
         ).count()
         + 1
     )
-    source_hash = hashlib.sha256()
-    for chunk in submission.source_file.chunks():
-        source_hash.update(chunk)
-    submission.source_file.seek(0)
+    with submission.source_file.read_using_cache() as source_file:
+        source_hash = hashlib.sha256()
+        for chunk in source_file.chunks():
+            source_hash.update(chunk)
 
     proof_data = {
         'id': submission.id,

oioioi/mailsubmit/utils.py

@@ -74,10 +74,9 @@ def accept_mail_submission(request, mailsubmission):
 
 def mail_submission_hashes(mailsubmission):
     source_hash = hashlib.sha256()
-    for chunk in mailsubmission.source_file.chunks():
+    for chunk in mailsubmission.source_file.read_using_cache().chunks():
         source_hash.update(chunk)
     source_hash = source_hash.hexdigest()
-    mailsubmission.source_file.seek(0)
 
     pi = mailsubmission.problem_instance