A new table is created to track which path_id attachments are images,
and for those their metadata, and which thumbnails have been created.
Using path_id as the effective primary key lets us ignore if the
attachment is archived or not, saving some foreign key messes.
A new worker is added to observe events when rows are added to this
table, and to generate and store thumbnails for those images in
differing sizes and formats.
Previously, tests that exercised code paths that added local
uploads did not always clean up `settings.LOCAL_UPLOADS_DIR`
after the test was complete.
Updates the `ZulipTestCase` class to remove any local uploads
in the unique `settings.LOCAL_UPLOADS_DIR` in `tearDown` for
all tests.
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.
Alex Vandiver
Vector73
Anders Kaseorg
AcKindle3
Lauryn Menard