Because loading boto3 is so slow, this saves a significant amount of
time (0.3s or so) in process startup on servers which are not using
the S3 file storage backend.
Fixes#25414.
We add Attachment.scheduled_messages relation to track ScheduledMessages
which reference the attachment.
The import bits can be done after merging this, by updating #25345.
Actions like deleting realms may leave unreferenced uploads in the
attachment storage backend.
Fix these by walking the complete contents of the attachment storage
backend, and removing files which are no longer present in the
database. This may take quite some time, as it is necessarily O(n) in
the number of files uploaded to the system.
Importing `upload_backend` directly means that in testing it must also
be mocked where it is imported, in order to correctly test the right
backend. Since `get_avatar_url` is part of the public
`ZulipUploadBackend` API, add another helper method to call that.
Alex Vandiver
Anders Kaseorg
Mateusz Mandera