Create a job to handle the migration of Ralph to Vector

[bmtcril]

As part of moving to Vector as the default event pipeline we should offer operators a way to easily port their events to the new xAPI database. This can be a "do" script, it will need instructions and documentation. The migration should look like either:

Change the default Vector database to be the existing xapi database

• Upgrade the Aspects plugin
• Update plugin setting ASPECTS_VECTOR_DATABASE to xapi
• Save config / do init
• (I think that's it?)

Use the openedx database for Vector

• Upgrade the Aspects plugin
• Save config / do init
• Run a migration script that does essentially INSERT INTO openedx.xapi_events_all SELECT * from xapi.xapi_events_all, possibly in smaller chunks, ordered by date, so as not to take down a live service