Render the has_one/has_many associations on the edit page

[adrianthedev]

We have some feedback from a content team (non-technical users of the app) that they would like the has_many associations in the edit page.

- It seems that they quite often dont understand the difference between "edit" and "view".
So when they click on edit, they are wondering where the "has_many" resources are and ask me then. After some time you get more used to it, but it still feels a bit strange to them

[Paul-Bob]

field :user, as: :has_one, show_on: :edit
field :users, as: :has_many, show_on: :edit

FUN FACT: This was my first avo PR #880 🥲