A proof-of-concept implementation of an ACL system is available in
`lp:~bjornt/launchpad/privacy-spike`. We're going to use that to
implement ACLs for bugs. The reason we're doing this is to get something
landed, so that we can test things using real data and use cases.

The first phase will consist of replacing the `BugSubscription` table
with the `BugTaskACL` table when doing permission checking. This change
won't be visible to the users, everything is done under the hood,
keeping the existing UI and functionality. It will still be required to
get subscribed to a private bug in order to see it.

These are the list of tasks that need to happen. Each task should be
possible to implement and land in its own branch. They don't necessarily
need to happen in the order below, but some tasks do depend on other
tasks.

    * '''Add the `BugTaskACL` table''' - get the db patch and model
    class landed.
    * '''Add `the IACL` interface and adapter, etc.''' - add the general
    ACL infrastructure that is needed, like the `IACL` interface and
    base adapter, the `ACLObjectType` enum, `__acl_class__` attribute,
    etc.
    * '''Populate BugTaskACL table and keep it up-to-date'''  when
    subscribing/unsubcribing, and changing privacy, the `ACLs` for the
    bugtasks should be updated to reflect the subscriber list if the bug
    is private, or to be the public ACL if it's public. We will have to
    update the ACLs for all the bug tasks, since the subscription is per
    bug, not per bug task.
    * '''Use the `BugTaskACL` table instead of the `BugSubscription`
    table''' - to check whether the user has access to view the bug.
    Before doing this, testing should be performed on staging, to ensure
    that we don't have a performance regression.
    * '''Add `BugTask.private` and write a trigger to keep it
    up-to-date''' - to keep queries for public bugs fast, we need to
    have an attribute on the bugtask to indicate whether it's public or
    private (public is the common case and should be fast). We don't
    need to use the attribute yet, since we already check `Bug.private`.
    * '''Use `BugTask.private` instead of `Bug.private` in permission
    queries''' - we want security to be on the bugtask level, not on the
    bug level. We should update all places where we use `Bug.private` to
    use `BugTask.private` wehre possible. It might not be possible to
    update it everywhere, until we have the UI specification ready. For
    this reason, it might makes sense to not do any work at all on this
    task, before the UI is defined.