Automatically assign a valuator depending on the category

[microstudi]

Following #25, we need to automatically assign all the valuators assigned in a category as a valuators of any new proposal created in that category.
The assignation must behave exactly as a manual assignation, the same email should be received by the assigned valuator when the proposal is published.

• When a proposal is published under a certain category, any valuators listed in it (see #25), should be assigned automatically.
• When the valuator is assigned, it should receive the same email as if it was assigned manually.
• Tests