OPP -> autocomplete fields in viewpoint creation and update form

[submarcos]

as described here : Terralego/terra-opp#24

Terra-admin should provide way to set new city or theme, and not be limit by existing list (-> circular problem : no viewpoint / no value because admin deals with SELECT instead of input text, and values come from existing viewpoints properties) -> should be fixed in admin with autocomplete and free text field to let user to choose existing or set new value

• Firstly, we need to replace select input for city and theme with an autocomplete / free input text
• In the future, may be these list should be managed in custom models and menu, with proper api endpoint. This would provide a way to define Fixed list available without viewpoint data