Rework measurements - #2476

This can happen if e.g. the user deleted their user and is still logged in
on the app

…h-sync)(wger#2328)

Changes:
* Schema: Added 'is_official' flag and UniqueConstraint to measurements.Category.
* Migrations: Created schema migration (0007) and data backfill migration (0008).
* API (Serializer/Views): Re-pointed WeightEntry endpoint to read/write from Measurement models using the official 'body_weight' category.
* API (Filters): Updated WeightEntryFilterSet to map legacy 'weight' params to the new 'value' column.
* Legacy: Marked WeightEntry model as deprecated.

This is not needed, as externally synced entries can be found through other
fields, such as the external_id

Measurement categories can optionally point to another category, In practice, this
will just be something like blood pressure since these values only make sense in a
pair. Others can be interesting while combined (heart rate and steps), but are valid
on their own.

# Conflicts:
#	wger/utils/viewsets.py

Users can enter values in mixed units, external syncs will store their
provenance here as well

…ts-and-weight-2328

refactor(weight): migrate WeightEntry to Measurement (wger#2328)

This was forgotten in the previous commits