Fixes#2665.
Regenerated by tabbott with `lint --fix` after a rebase and change in
parameters.
Note from tabbott: In a few cases, this converts technical debt in the
form of unsorted imports into different technical debt in the form of
our largest files having very long, ugly import sequences at the
start. I expect this change will increase pressure for us to split
those files, which isn't a bad thing.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
We send add events on upload, update events when sending a message
referencing it, and delete updates on removal.
This should make it possible to do real-time sync for the attachments
UI.
Based in part on work by Aastha Gupta.