Fix bugs in the download script

[louis-hildebrand]

• It seems like the download script always thinks assets are duplicates of themselves, since I forgot to exclude the file itself when searching
• The script should be much clearer about why certain files were or were not downloaded. When the script finishes, it should display, for each announcement:
  • The path to the file, if it was downloaded successfully and is not a duplicate
  • That the file was a duplicate and which old file it's a duplicate of, if applicable
  • That there was an error, if applicable
• The announcements video should have a date in its name to avoid confusion between videos from different weeks
• Maybe there should be more tests for the download script