Consolidate model

[samuel-iseli]

• review model
• consider ideas from samuel-iseli/depotcommun

[samuel-iseli]

I think we should discuss a bit about the model. It seems too complicated to me for a start.

• do we really need multiple depots? it adds quite some complexity and makes it hard to implement a frontend app without configuration (there is not generic url for interacting with the back-end).
• The current deployment problems seem to have something to do with the multi-multi relation between depots and users
• I would remove the depot entity for starters. We can introduce the concept of multiple depots when we really need it.
• Why do we need our own uuid generator, I thought django has auto-generated id fields out of the box?

As for the current deployment errors: perhaps it helps to delete the database and let migrations create a new one.