Add areas to initiatives

Is your feature request related to a problem?

Initiatives cannot be associated to an area like other participatory spaces.

Describe the solution you'd like

As a user I want to be able to associate an area to initiatives during my initiative creation and when editing my initiative in the back office.

As a user I want to be able to filter initiatives in the index by area using a dropdown menu.

Full spec here : https://github.com/OpenSourcePolitics/dev/issues/32

Describe alternatives you've considered

None

Does this issue could impact on users private data?

No