Migrations: Move migration notebooks out of the demo directory

[eecavanna]

The migration notebooks currently live in a place where I plan to eventually remove them from, which is the "demo/metadata_migration/notebooks" directory.

The reason they are there is that, when I created the first one (last August), I saw that there was already a notebook related to migrating Mongo data in that part of the repo and so I put the one I was working on there alongside it. Now that the migration notebooks have become a standard part of our migration process, I no longer want them to be in a directory called "demo" (which I assume its namer used to mean, "demonstration").

I have been procrastinating on moving them out of there. Once the Berkeley Schema Roll Out is done, I plan to move them to somewhere else in the repo (TBD).

[eecavanna]

Done via PR #813.