Persist announcements

[rcthomas]

This allows announcements to be initialized from a previous list of persisted announcements and persist the list of announcements on update. We just use a JSON file for the storage.

• Add configuration options for using persistent announcements
• Add logging though it's a bit weird with traitlets
• Change "storage" to "queue" and call it that throughout
• Make the persisted announcement queue be the storage mechanism
• Change template accordingly
• Increment version number