Previously, Attachment.is_realm_public and its cousin,
Attachment.is_web_public, were properties that began as False and
transitioned to True only when a message containing a link to the
attachment was sent to the appropriate class of stream, or such a link
was added as part of editing a message.
This pattern meant that neither field was updated in situations where
the access permissions for a message changed:
* Moving the message to a different stream.
* Changing the permissions for a stream containing links to the message.
This correctness issue has limited security impact, because uploaded
files are secured both by a random URL and by these access checks.
To fix this, we reformulate these fields as a cache, with code paths
that change the permissions affecting an attachment responsible for
setting these values to the `None` (uncached) state. We prefer setting
this `None` state over computing the correct permissions, because the
correct post-edit permissions are a function of all messages
containing the attachment, and we don't want to be responsible for
fetching all of those messages in the edit code paths.
Tim Abbott