TODO before 0.12.0 release:

* ~~Support importing Pebble Health data from old database~~ DONE, needs check.
* ~~Add onboarding activity on first startup (to merge old data)~~ DONE, needs check.
* Rename ActivitySample to Sample or something similar, and also related methods.
* Reconsider DBHelper.getUser() (multi user in GB does not make much sense)

Non blocking issues:

* Add back UUID_CHARACTERISTIC_PAIR support, at least optionally
* CSV Export