Use project actions to cleanup empty transient projects

[brollb]

Currently, cleaned up transient projects only delete the metadata from the
database. Since they don't use the official delete_project method, they don't
do a few things including:

• delete content from s3
• clear the project cache

The same is true of projects deleted automatically by MongoDB using the
deleteAt field. This index should be removed and deletion should be handled by
the system to ensure the aforementioned issues don't emerge there, too.