Add before hooks

[Flo0807]

Backpex provides hooks that are called after changes are persisted to the database: https://hexdocs.pm/backpex/hooks.html

We could also add Hooks that are called before changes are persisted.

• on_item_creating
• on_item_deleting
• on_item_updating

These hooks might be used to add additional changes.

We need to discuss what parameters these hooks receive, what they return and what Backpex does with the result.