uploads: don't clean up replaced files too early.

Don't try delete files belonging to completed uploads during pruning. If
an uploaded post was replaced shortly after upload, then the original
file could get deleted during pruning since it's no longer in use, but
this isn't supposed to happen until the replacement grace period (30
days) is over.

app/models/upload.rb

@@ -101,7 +101,7 @@ class Upload < ApplicationRecord
 
     def delete_files
       # md5 is blank if the upload errored out before downloading the file.
-      if md5.blank? || Upload.where(md5: md5).exists? || Post.where(md5: md5).exists?
+      if is_completed? || md5.blank? || Upload.where(md5: md5).exists? || Post.where(md5: md5).exists?
         return
       end