Several tasks are explicitly named as (or like)
their old, deprecated import path.

The issue here is that django-user-tasks listens for task
invocations, and attempts to import the task based on its name.
If the task name is completely wrong, user-tasks will catch
the ImportError and move on.
If the task is a valid *deprecated* import, though, then
user-tasks will choke on the raised `DeprecatedEdxPlatformImportError`.

Thus, we must rename three tasks to their new full path:
1. entitlements.expire_old_enrollments
2. third_party_auth.fetch_saml_metadata
3. student.send_activation_email

The first two are run daily, and so are safe to be
renamed in place.

The third task must be renamed using an expand-contract
pattern; otherwise, we would drop hundreds of tasks
during the App vs. Worker out-of-sync version window
that happens at deployments.
This commit is the expand phase.