Migrate legacy records to the cloud

Resolves #12

**STILL A WIP, DO NOT MERGE**

- [x] Assemble record arrays from old data and split into chunks based on predefined query limits
- [ ] Use API endpoint for bulk ingestion of record chunks, see https://github.com/wp-stream/stream-node/issues/13
- [x] Display admin notice that a database migration is necessary
- [x] Initiate the migration with a button in the admin notice
- [ ] Show a progress bar that updates via ajax as it sends record chunks to the cloud