Make `Reservoir` optional from `Area` domain.

[bepitulaz]

In the current version of Tania, users must have a reservoir to create an area. This will become a problem for users who take their irrigation from unmeasurable (difficult to measure) sources such as a dam, tap water, and well.

This change will affect the backend and the front-end.

You can start the enhancement from Area domain https://github.com/Tanibox/tania-core/blob/master/src/assets/domain/area.go and the front-end https://github.com/Tanibox/tania-core/blob/master/resources/js/pages/intro/area.vue