Migration from the original heartbeat

[lmaotrigine]

Since this version makes some incompatible changes to the API and the internal storage model, a bit of work is necessary to migrate statistics and device and beat history from an existing server.

An implementation would involve

• Reading a dump.rdb and parsing out JSON, or require that a redis server be running
• Migrate devices to the new format using POST /api/devices and communicate the resulting tokens to the user.
• Migrate beat history ensuring foreign key constraints aren't violated.

cc @5HT2 since you wanted to take this up.

[lmaotrigine]

Today was a particularly dull day so I put this together in about 30 minutes. Feedback welcome.

[5HT2]

hey there, thanks for the ping + migration tool to look at c:
i was a lil busy recently, i'll review this tmrw most likely! tyvm