Make attachment downloads asynchronous (through notification)

[alesan99]

Fixes #6947
Extension of #6625

Clicking Download All on an attachment gallery will give you a download link in a notification once the download is ready.

Should fix the 504 Gateway Time-out error on large downloads.
This allows for downloads that take over a minute to download, given that the server is configured to have enough storage to store the file.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests
• Add a reverse migration if a migration is present in the PR

Testing instructions

• Click on the attachment gallery button on a record set with attachments
  
• Click "Download All"
• Verify that a "Attachment Archive Being Created" dialog appears after clicking download all.
• See that a notification appears with your download.
• Verify the notification is deleted after you click download (The file from the backend is deleted after you download it).
• Verify the download worked.
• Verify you still can't download all attachments in "browse in forms" if you haven't loaded in all the attachments.
• The Download All button should still work in general.
• Dev testing note:
  • Deleting a notification also deletes the file. You can verify this PR is working correctly by checking the zip files in DEPOSITORY_DIR

[alesan99]

NOTE: #6625 now solves #6947 . That means this PR is only good for making attachment downloads use notifications.

Benefits:

• Allows the user to navigate away from the download while the zip file is being prepared.
• Adds functionality for notifications to delete their associated files. Useful for cleaning up csv exports.

Downsides:

• Reintroduces Out of space error when downloading many attachments with "download all" #6403. This PR cannot support large attachment downloads unless the server is configured to have many GBs of free data.